Migrate GenerateBootstrapper to multithreaded execution

[jankratochvilcz]

Background

GenerateBootstrapper produces ClickOnce setup.exe / bootstrapper artifacts by delegating to the internal BootstrapperBuilder. It exposes many path inputs and an OutputPath that defaults to Directory.GetCurrentDirectory(), both of which break under multithreaded execution.

Migration scope

• Apply [MSBuildMultiThreadableTask]
• Implement IMultiThreadableTask
• Absolutize paths via TaskEnvironment.GetAbsolutePath() for: BootstrapperItems (ItemSpec), ApplicationFile, BootstrapperKeyFile, ComponentsLocation, OutputPath, Path, SupportUrl-derived paths
• Use TaskEnvironment.GetEnvironmentVariable() for: N/A (delegated builder may need review)
• Use TaskEnvironment.GetProcessStartInfo() for: N/A

Known complexity factors

• Default OutputPath = Directory.GetCurrentDirectory() must be re-derived from TaskEnvironment.ProjectDirectory
• Delegates heavy work to BootstrapperBuilder (under src/Tasks/BootstrapperUtil/) which performs its own file I/O and XML reads — propagate TaskEnvironment or absolutize before passing
• Sets [Output] BootstrapperComponentFiles/BootstrapperKeyFile from internal builder — preserve original-form values (Sin 1)
• Windows-only, NETFRAMEWORK-only feature path

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