Migrate WinMDExp to multithreaded execution

[jankratochvilcz]

Background

WinMDExp wraps the Windows Runtime metadata exporter winmdexp.exe to produce .winmd files from a managed assembly. As a ToolTask, its process launch and timestamp probes must move to TaskEnvironment.

Migration scope

• Apply [MSBuildMultiThreadableTask]
• Implement IMultiThreadableTask
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: WinMDModule, References items, OutputWindowsMetadataFile, OutputDocumentationFile, InputDocumentationFile, InputPDBFile, OutputPDBFile, SdkToolsPath, ToolPath
• Use TaskEnvironment.GetEnvironmentVariable() for: any SDK lookup fallbacks (review GenerateFullPathToTool)
• Use TaskEnvironment.GetProcessStartInfo() for: winmdexp.exe invocation

Known complexity factors

• ToolTask base — process invocation routes through TaskEnvironment.GetProcessStartInfo()
• Up-to-date check uses NativeMethodsShared.GetLastWriteFileUtcTime on multiple paths — must be absolute
• Sets [Output] OutputWindowsMetadataFile derived from input — preserve original spec (Sin 1)
• Windows-only feature

Exit criteria

• Build clean, no new warnings
• Existing tests pass
• No regression in error message paths (Sin 2 audit)
• No leakage of absolutized paths into [Output] properties (Sin 1 audit)

References

• Parent: Migrate Tasks in the MSbuild repo to the new Task type #11834
• Migration skill: .github/skills/multithreaded-task-migration/SKILL.md