Migrate UnregisterAssembly to multithreaded execution

[jankratochvilcz]

Background

UnregisterAssembly reverses RegisterAssembly by unregistering managed assemblies and their type libraries from COM. Like its counterpart, it is AppDomainIsolatedTaskExtension, gated on NETFRAMEWORK && FEATURE_APPDOMAIN.

Migration scope

• Apply [MSBuildMultiThreadableTask] (defensive)
• Implement IMultiThreadableTask only if reachable from a multithreaded host
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: Assemblies items, TypeLibFiles items, AssemblyListFile
• Use TaskEnvironment.GetEnvironmentVariable() for: N/A
• Use TaskEnvironment.GetProcessStartInfo() for: N/A

Known complexity factors

• AppDomain isolation; TaskEnvironment must cross the AppDomain boundary
• Native COM unregistration; mutates registry (TYPE_E_REGISTRYACCESS handling)
• NETFRAMEWORK-only — multithreaded mode unlikely to ever exercise this code
• Globally serialized in practice due to registry mutation

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