Enlighten ResolveKeySource task for multithreaded mode

[jankratochvilcz]

Enlighten ResolveKeySource task for multithreaded mode

Parent: #11834

Context

ResolveKeySource uses File.OpenRead(KeyFile) to read PFX key files. The KeyFile path could be relative, resolving against Environment.CurrentDirectory under multithreaded execution.

Sin 1 risk: ResolvedKeyFile = KeyFile assigns the input path directly to an [Output] property. After absolutization, we must use the original path for the output to avoid contamination.

Approach

1. Mark ResolveKeySource with [MSBuildMultiThreadableTask] and implement IMultiThreadableTask
2. Absolutize KeyFile via TaskEnvironment.GetAbsolutePath() before File.OpenRead()
3. Preserve original KeyFile string for ResolvedKeyFile output property (Sin 1)
4. Use OriginalValue in error messages (Sin 2)

Acceptance criteria

• ResolveKeySource decorated [MSBuildMultiThreadableTask] and implements IMultiThreadableTask
• KeyFile absolutized before File.OpenRead()
• [Output] ResolvedKeyFile preserves original path string (Sin 1)
• Error messages use original paths (Sin 2)
• All existing tests pass
• No new compiler warnings

References

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