Write ExpectedExitCode marker in GenerateMarkerFiles

[Copilot]

GenerateMarkerFiles did not emit the expected-exit-code marker, so merged/standalone test payloads could miss ExpectedExitCode.txt. This change adds marker generation directly in the existing marker pipeline.

• Scope

  • Updates src/tests/Directory.Build.targets in GenerateMarkerFiles.

• Behavior change

  • Adds a marker entry for $(IntermediateOutputPath)\ExpectedExitCode.txt.
  • Writes $(ExpectedExitCode) as file content.
  • Applies only when:
    • $(ExpectedExitCode) is set
    • test is merged runner or standalone runner
    • $(CLRTestKind) is not SharedLibrary

• Implementation (minimal diff)

  <MarkerFile Include="$(IntermediateOutputPath)\ExpectedExitCode.txt"
              Condition="'$(ExpectedExitCode)' != '' and ('$(IsMergedTestRunnerAssembly)' == 'true' or '$(BuildAllTestsAsStandalone)' == 'true') and '$(CLRTestKind)' != 'SharedLibrary'">
    <Type>$(ExpectedExitCode)</Type>
  </MarkerFile>

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[Copilot]

Pull request overview

Adds generation of the ExpectedExitCode.txt marker during GenerateMarkerFiles so merged/standalone test payloads reliably carry the expected exit code alongside other marker files.

Changes:

• Adds a MarkerFile item for $(IntermediateOutputPath)\ExpectedExitCode.txt gated on $(ExpectedExitCode) being set and the build producing a merged or standalone runner (excluding SharedLibrary).
• Writes $(ExpectedExitCode) as the marker file content via the existing WriteLinesToFile marker pipeline.