Add L0 tests for REPL parser and session routing

Parser tests (DapReplParserL0, 22 tests):
- help: bare, case-insensitive, with topic
- run: simple script, with shell, env, working_directory, all options
- Edge cases: escaped quotes, commas in env values
- Errors: empty args, unquoted arg, unknown option, missing paren
- Non-DSL input falls through: expressions, wrapped expressions, empty
- Help text contains expected commands and options
- Internal helpers: SplitArguments with nested braces, empty env block

Session integration tests (DapDebugSessionL0, 4 tests):
- REPL help returns help text
- REPL non-DSL input falls through to expression evaluation
- REPL parse error returns error result (not a DAP error response)
- watch context still evaluates expressions (not routed through REPL)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Test/L0/Worker/DapDebugSessionL0.cs

@@ -968,5 +968,171 @@ namespace GitHub.Runner.Common.Tests.Worker
         }
 
         #endregion
+
+        #region REPL routing tests
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ReplHelpReturnsHelpText()
+        {
+            using (CreateTestContext())
+            {
+                await InitializeSessionAsync();
+
+                var evaluateJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 10,
+                    Type = "request",
+                    Command = "evaluate",
+                    Arguments = Newtonsoft.Json.Linq.JObject.FromObject(new EvaluateArguments
+                    {
+                        Expression = "help",
+                        Context = "repl"
+                    })
+                });
+                _sentResponses.Clear();
+                await _session.HandleMessageAsync(evaluateJson, CancellationToken.None);
+
+                Assert.Single(_sentResponses);
+                Assert.True(_sentResponses[0].Success);
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ReplExpressionFallsThroughToEvaluation()
+        {
+            using (var hc = CreateTestContext())
+            {
+                await InitializeSessionAsync();
+                _session.HandleClientConnected();
+
+                var exprValues = new DictionaryContextData();
+                exprValues["github"] = new DictionaryContextData
+                {
+                    { "repository", new StringContextData("owner/repo") }
+                };
+
+                var step = CreateMockStepWithEvaluatableContext(hc, "Run tests", exprValues);
+                var jobContext = CreateMockJobContext();
+
+                var stepTask = _session.OnStepStartingAsync(step.Object, jobContext.Object, isFirstStep: true, CancellationToken.None);
+                await Task.Delay(100);
+
+                // In REPL context, a non-DSL expression should still evaluate
+                var evaluateJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 20,
+                    Type = "request",
+                    Command = "evaluate",
+                    Arguments = Newtonsoft.Json.Linq.JObject.FromObject(new EvaluateArguments
+                    {
+                        Expression = "github.repository",
+                        Context = "repl"
+                    })
+                });
+                _sentResponses.Clear();
+                await _session.HandleMessageAsync(evaluateJson, CancellationToken.None);
+
+                Assert.Single(_sentResponses);
+                Assert.True(_sentResponses[0].Success);
+
+                var continueJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 21,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await _session.HandleMessageAsync(continueJson, CancellationToken.None);
+                await Task.WhenAny(stepTask, Task.Delay(5000));
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task ReplParseErrorReturnsErrorResult()
+        {
+            using (CreateTestContext())
+            {
+                await InitializeSessionAsync();
+
+                // Malformed run() command
+                var evaluateJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 10,
+                    Type = "request",
+                    Command = "evaluate",
+                    Arguments = Newtonsoft.Json.Linq.JObject.FromObject(new EvaluateArguments
+                    {
+                        Expression = "run()",
+                        Context = "repl"
+                    })
+                });
+                _sentResponses.Clear();
+                await _session.HandleMessageAsync(evaluateJson, CancellationToken.None);
+
+                Assert.Single(_sentResponses);
+                Assert.True(_sentResponses[0].Success);
+                // The response is successful at the DAP level (not an error
+                // response), but the result body conveys the parse error
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public async Task WatchContextStillEvaluatesExpressions()
+        {
+            using (var hc = CreateTestContext())
+            {
+                await InitializeSessionAsync();
+                _session.HandleClientConnected();
+
+                var exprValues = new DictionaryContextData();
+                exprValues["github"] = new DictionaryContextData
+                {
+                    { "repository", new StringContextData("owner/repo") }
+                };
+
+                var step = CreateMockStepWithEvaluatableContext(hc, "Run tests", exprValues);
+                var jobContext = CreateMockJobContext();
+
+                var stepTask = _session.OnStepStartingAsync(step.Object, jobContext.Object, isFirstStep: true, CancellationToken.None);
+                await Task.Delay(100);
+
+                // watch context should NOT route through REPL even if input
+                // looks like a DSL command — it should evaluate as expression
+                var evaluateJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 20,
+                    Type = "request",
+                    Command = "evaluate",
+                    Arguments = Newtonsoft.Json.Linq.JObject.FromObject(new EvaluateArguments
+                    {
+                        Expression = "github.repository",
+                        Context = "watch"
+                    })
+                });
+                _sentResponses.Clear();
+                await _session.HandleMessageAsync(evaluateJson, CancellationToken.None);
+
+                Assert.Single(_sentResponses);
+                Assert.True(_sentResponses[0].Success);
+
+                var continueJson = JsonConvert.SerializeObject(new Request
+                {
+                    Seq = 21,
+                    Type = "request",
+                    Command = "continue"
+                });
+                await _session.HandleMessageAsync(continueJson, CancellationToken.None);
+                await Task.WhenAny(stepTask, Task.Delay(5000));
+            }
+        }
+
+        #endregion
     }
 }

src/Test/L0/Worker/DapReplParserL0.cs

@@ -0,0 +1,314 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using GitHub.Runner.Common.Tests;
+using GitHub.Runner.Worker.Dap;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Worker
+{
+    public sealed class DapReplParserL0
+    {
+        #region help command
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_HelpReturnsHelpCommand()
+        {
+            var cmd = DapReplParser.TryParse("help", out var error);
+
+            Assert.Null(error);
+            var help = Assert.IsType<HelpCommand>(cmd);
+            Assert.Null(help.Topic);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_HelpCaseInsensitive()
+        {
+            var cmd = DapReplParser.TryParse("Help", out var error);
+            Assert.Null(error);
+            Assert.IsType<HelpCommand>(cmd);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_HelpWithTopic()
+        {
+            var cmd = DapReplParser.TryParse("help(\"run\")", out var error);
+
+            Assert.Null(error);
+            var help = Assert.IsType<HelpCommand>(cmd);
+            Assert.Equal("run", help.Topic);
+        }
+
+        #endregion
+
+        #region run command — basic
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunSimpleScript()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo hello\")", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("echo hello", run.Script);
+            Assert.Null(run.Shell);
+            Assert.Null(run.Env);
+            Assert.Null(run.WorkingDirectory);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithShell()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo hello\", shell: \"bash\")", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("echo hello", run.Script);
+            Assert.Equal("bash", run.Shell);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithWorkingDirectory()
+        {
+            var cmd = DapReplParser.TryParse("run(\"ls\", working_directory: \"/tmp\")", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("ls", run.Script);
+            Assert.Equal("/tmp", run.WorkingDirectory);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithEnv()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo $FOO\", env: { FOO: \"bar\" })", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("echo $FOO", run.Script);
+            Assert.NotNull(run.Env);
+            Assert.Equal("bar", run.Env["FOO"]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithMultipleEnvVars()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo\", env: { A: \"1\", B: \"2\" })", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal(2, run.Env.Count);
+            Assert.Equal("1", run.Env["A"]);
+            Assert.Equal("2", run.Env["B"]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithAllOptions()
+        {
+            var input = "run(\"echo $X\", shell: \"zsh\", env: { X: \"1\" }, working_directory: \"/tmp\")";
+            var cmd = DapReplParser.TryParse(input, out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("echo $X", run.Script);
+            Assert.Equal("zsh", run.Shell);
+            Assert.Equal("1", run.Env["X"]);
+            Assert.Equal("/tmp", run.WorkingDirectory);
+        }
+
+        #endregion
+
+        #region run command — edge cases
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithEscapedQuotes()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo \\\"hello\\\"\")", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("echo \"hello\"", run.Script);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunWithCommaInEnvValue()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo\", env: { CSV: \"a,b,c\" })", out var error);
+
+            Assert.Null(error);
+            var run = Assert.IsType<RunCommand>(cmd);
+            Assert.Equal("a,b,c", run.Env["CSV"]);
+        }
+
+        #endregion
+
+        #region error cases
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunEmptyArgsReturnsError()
+        {
+            var cmd = DapReplParser.TryParse("run()", out var error);
+
+            Assert.NotNull(error);
+            Assert.Null(cmd);
+            Assert.Contains("requires a script argument", error);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunUnquotedArgReturnsError()
+        {
+            var cmd = DapReplParser.TryParse("run(echo hello)", out var error);
+
+            Assert.NotNull(error);
+            Assert.Null(cmd);
+            Assert.Contains("quoted string", error);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunUnknownOptionReturnsError()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo\", timeout: \"10\")", out var error);
+
+            Assert.NotNull(error);
+            Assert.Null(cmd);
+            Assert.Contains("Unknown option", error);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_RunMissingClosingParenReturnsError()
+        {
+            var cmd = DapReplParser.TryParse("run(\"echo\"", out var error);
+
+            Assert.NotNull(error);
+            Assert.Null(cmd);
+        }
+
+        #endregion
+
+        #region non-DSL input falls through
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_ExpressionReturnsNull()
+        {
+            var cmd = DapReplParser.TryParse("github.repository", out var error);
+
+            Assert.Null(error);
+            Assert.Null(cmd);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_WrappedExpressionReturnsNull()
+        {
+            var cmd = DapReplParser.TryParse("${{ github.event_name }}", out var error);
+
+            Assert.Null(error);
+            Assert.Null(cmd);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void Parse_EmptyInputReturnsNull()
+        {
+            var cmd = DapReplParser.TryParse("", out var error);
+            Assert.Null(error);
+            Assert.Null(cmd);
+
+            cmd = DapReplParser.TryParse(null, out error);
+            Assert.Null(error);
+            Assert.Null(cmd);
+        }
+
+        #endregion
+
+        #region help text
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void GetGeneralHelp_ContainsCommands()
+        {
+            var help = DapReplParser.GetGeneralHelp();
+
+            Assert.Contains("help", help);
+            Assert.Contains("run", help);
+            Assert.Contains("expression", help, System.StringComparison.OrdinalIgnoreCase);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void GetRunHelp_ContainsOptions()
+        {
+            var help = DapReplParser.GetRunHelp();
+
+            Assert.Contains("shell", help);
+            Assert.Contains("env", help);
+            Assert.Contains("working_directory", help);
+        }
+
+        #endregion
+
+        #region internal parser helpers
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void SplitArguments_HandlesNestedBraces()
+        {
+            var args = DapReplParser.SplitArguments("\"hello\", env: { A: \"1\", B: \"2\" }", out var error);
+
+            Assert.Null(error);
+            Assert.Equal(2, args.Count);
+            Assert.Equal("\"hello\"", args[0].Trim());
+            Assert.Contains("A:", args[1]);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Worker")]
+        public void ParseEnvBlock_HandlesEmptyBlock()
+        {
+            var result = DapReplParser.ParseEnvBlock("{ }", out var error);
+
+            Assert.Null(error);
+            Assert.NotNull(result);
+            Assert.Empty(result);
+        }
+
+        #endregion
+    }
+}