Enlighten SignFile task for multithreaded mode

[jankratochvilcz]

Enlighten SignFile task for multithreaded mode

Parent: #11834

Context

SignFile passes SigningTarget.ItemSpec to SecurityUtilities.SignFile() which performs file I/O. Under multithreaded execution, relative paths would resolve against Environment.CurrentDirectory (shared process state) rather than the project directory.

Approach

1. Mark SignFile with [MSBuildMultiThreadableTask] and implement IMultiThreadableTask with public TaskEnvironment TaskEnvironment { get; set; } = TaskEnvironment.Fallback;
2. Absolutize SigningTarget.ItemSpec via TaskEnvironment.GetAbsolutePath() before passing to SecurityUtilities.SignFile()
3. Use OriginalValue in error messages to avoid path inflation (Sin 2)

Acceptance criteria

• SignFile decorated [MSBuildMultiThreadableTask] and implements IMultiThreadableTask
• SigningTarget.ItemSpec absolutized before SecurityUtilities.SignFile() call
• Error messages use original path strings (Sin 2)
• No AbsolutePath leak into user-visible output (Sin 1)
• 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