Migrate TlbImp to multithreaded execution

[jankratochvilcz]

Background

TlbImp (internal nested ToolTask used by ResolveComReference) wraps TlbImp.exe to import a COM type library into a managed interop assembly. As a ToolTask, its process invocation depends on global process state.

Migration scope

• Apply [MSBuildMultiThreadableTask] to TlbImp (audit shared AxTlbBaseTask base)
• Implement IMultiThreadableTask
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: TypeLibName, AssemblyFile/OutputAssembly, KeyFile, KeyContainer-derived paths, ReferenceFiles, ToolPath
• Use TaskEnvironment.GetEnvironmentVariable() for: env vars consumed by ToolTask base (review)
• Use TaskEnvironment.GetProcessStartInfo() for: TlbImp.exe invocation

Known complexity factors

• Shares AxTlbBaseTask : ToolTaskExtension with AxImp — coordinate base-class refactor
• ToolTask process launch must route through TaskEnvironment.GetProcessStartInfo()
• Driven by ResolveComReference; that call site must propagate TaskEnvironment
• Windows-only / NETFRAMEWORK-only feature

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