Migrate ResolveSDKReference to multithreaded execution

[jankratochvilcz]

Background

ResolveSDKReference resolves SDK references against installed Windows / extension SDKs and emits resolved metadata items. It is one of the larger tasks (~75 KB) with substantial caching and Windows-specific logic.

Migration scope

• Apply [MSBuildMultiThreadableTask]
• Implement IMultiThreadableTask
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: SDKReferences items, References items, Projects items, ResolvedSDKReferences ItemSpecs, RuntimeReferenceOnlySDKDependencies, ProjectName-derived paths
• Use TaskEnvironment.GetEnvironmentVariable() for: any registry/SDK fallbacks invoked from ToolLocationHelper (review)
• Use TaskEnvironment.GetProcessStartInfo() for: N/A

Known complexity factors

• Several static readonly caches/regexes (PlatformAliases, SdkReferenceFormatRegex, s_appxSplitChar, s_defaultTargetPlatformVersion) — confirm all are immutable / thread-safe
• Calls into ToolLocationHelper and SDK manifest parsing — those helpers may consume process env (PATH, registry) outside TaskEnvironment
• Windows-only main path; NativeMethodsShared.IsWindows early-out
• Emits many [Output] ItemGroups derived from ItemSpecs — preserve original-form (Sin 1)

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