Migrate AxImp to multithreaded execution

[jankratochvilcz]

Background

AxImp (internal nested ToolTask used by ResolveComReference) wraps AxImp.exe to generate ActiveX wrapper assemblies. As a ToolTask, it inherits process-launch semantics that depend on the current working directory and environment.

Migration scope

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

Known complexity factors

• Inherits AxTlbBaseTask : ToolTaskExtension — shared base with TlbImp; migration must coordinate
• ToolTask Execute constructs ProcessStartInfo internally — likely needs override of ExecuteTool/GenerateCommandLineCommands paths to route through TaskEnvironment
• Used as a sub-task by ResolveComReference; verify call sites pass through 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