Add test coverage for custom groups and fail with ERROR_NOT_SUPPORTED…

[OneBlue]

… for root ns processes

Summary of the Pull Request

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[OneBlue]

We can add support later if there's a usecase for it. For now let's fail with a proper error code

[Copilot]

Pull request overview

This PR improves WSLA coverage and behavior around custom user/group handling by adding tests for specifying a custom group in containers/exec, and by making root-namespace process creation explicitly fail when a custom user is requested.

Changes:

• Added a root-namespace-process negative test validating SetUser(...) fails with ERROR_NOT_SUPPORTED.
• Added test coverage verifying user:group is applied correctly for container init processes and exec.
• Added a service-side guard returning ERROR_NOT_SUPPORTED when WSLA_PROCESS_OPTIONS::User is specified for root-namespace processes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
test/windows/WSLATests.cpp	Adds new tests for custom group wiring (container init + exec) and an error-path test for root-namespace processes.
src/windows/wslaservice/exe/WSLAVirtualMachine.cpp	Enforces ERROR_NOT_SUPPORTED when Options.User is provided for root-namespace process creation.