Migrate GetAssembliesMetadata to multithreaded execution

[jankratochvilcz]

Migrates GetAssembliesMetadata to support MSBuild's multithreaded execution model.

Changes

• [MSBuildMultiThreadableTask] applied; class implements IMultiThreadableTask with TaskEnvironment = TaskEnvironment.Fallback default.
• Each assembly file path absolutized via TaskEnvironment.GetAbsolutePath before MetadataReader-based access (AssemblyInformation opens the file via File.OpenRead / OpenScope / native metadata reads, all of which need an absolute path on a thread node).

Compatibility audit

Ran the 6-deadly-sins playbook (see .github/skills/multithreaded-task-migration/SKILL.md):

• Sin 1 ([Output] contamination): AssembliesMetadata[i].ItemSpec is sourced from AssemblyAttributes.AssemblyFullPath, which AssemblyInformation initializes from the path passed into its constructor. To avoid leaking the absolutized path into the output items (which would change the user-observable ItemSpec), we overwrite attributes.AssemblyFullPath with absoluteAssemblyPath.OriginalValue before constructing the TaskItem.
• Sin 2 (error message inflation): N/A — task logs no errors/warnings/messages that include the path.
• Sin 3 (?? swallowing exceptions): N/A — no null-coalescing added.
• Sin 4 (try-catch scope): N/A — no try/catch in Execute.
• Sin 5 (canonicalization): N/A — no dictionary/set keyed on the path; FileSystems.Default.FileExists does not require canonical form.
• Sin 6 (empty/null inputs): Original code passed empty/null entries to FileSystems.Default.FileExists which silently returned false and skipped them; GetAbsolutePath("") would now throw ArgumentException and abort the entire batch. Guarded with string.IsNullOrEmpty(assemblyPath) -> continue to preserve original skip-silently semantics.

Validation

• Microsoft.Build.Tasks.csproj builds clean (0 warnings, 0 errors).
• All GetAssembliesMetadata-filtered tests pass (2/2 on net472 — the test class is #if NETFRAMEWORK-gated, so net10.0 has no tests for this task).

Part of #11834. Closes #13570.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates GetAssembliesMetadata to be MSBuild multi-threadable and improves path handling to avoid exceptions while preserving output item behavior.

Changes:

• Marked the task as multi-threadable and added TaskEnvironment support.
• Added guarding for null/empty assembly path entries before path resolution.
• Resolved assembly paths to absolute paths for file existence checks and metadata reading while attempting to preserve original output ItemSpec behavior.

[AlesProkop]

Consider adding a decoy test to expand test coverage.