Discourage changes/overrides to UserNamespace class

What does this MR do and why?

Because :namespace factory implicitly meant 'User' namespace, we want to ensure existing tests are still testing against the intended functionality.

We want to carefully preserve this intention by changing the class but also ensuring that any changes are carefully managed between ~"group::workspace" team and the domain experts of whatever functionality such a change might affect.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341447 (closed)