Migrate GenerateLauncher to multithreaded execution

[jankratochvilcz]

Background

GenerateLauncher produces a ClickOnce launcher executable via the internal LauncherBuilder. It accepts launcher binary location and entry point inputs that need to be absolutized for thread-safe execution.

Migration scope

• Apply [MSBuildMultiThreadableTask]
• Implement IMultiThreadableTask
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: LauncherPath, OutputPath, EntryPoint.ItemSpec, AssemblyName-derived paths
• Use TaskEnvironment.GetEnvironmentVariable() for: N/A
• Use TaskEnvironment.GetProcessStartInfo() for: N/A

Known complexity factors

• Delegates to LauncherBuilder (internal) which performs file copy / patching of the launcher binary
• Windows-only (NativeMethodsShared.IsWindows early-out)
• Sets [Output] OutputEntryPoint derived from input — preserve original spec (Sin 1)

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