migrate PickBestRid

[AlesProkop]

Summary

Migrates PickBestRid to MSBuild's multithreaded task execution model by opting into IMultiThreadableTask and routing path resolution through TaskEnvironment.

Changes

PickBestRid.cs

• Added [MSBuildMultiThreadableTask] and implemented IMultiThreadableTask with TaskEnvironment = TaskEnvironment.Fallback.
• Resolved RuntimeGraphPath via TaskEnvironment.GetAbsolutePath(...) before File.Exists and RuntimeGraphCache.GetRuntimeGraph.
• Kept the original RuntimeGraphPath in error messages (no absolutized leak).

GivenAPickBestRidMultiThreading.cs (new) — 4 correctness tests:

Test	Guards against
ItResolvesRelativeRuntimeGraphPathAgainstProjectDirectory	Path resolution falling back to process CWD.
MatchingRidOutputContainsOnlyRidNoPathPrefix	Absolutized path leaking into the MatchingRid output.
MissingFileErrorContainsOriginalRelativePathNotAbsolutized	Error-message path inflation.
ItProducesIdenticalOutcomeInFallbackAndIsolatedModes_Success	Behavior parity between Fallback and isolated TaskEnvironment.

Validation

• MSBuild Thread-Safe Task Analyzer: 0 diagnostics on PickBestRid.cs.
• New tests: 4/4 pass. Existing GivenAPickBestRid tests: 6/6 still pass.