Fix test environment pollution in spec/lib/gitlab/request_context_spec.rb

What does this MR do?

Closes #58969 (closed)

What are the relevant issue numbers?

Prevents a spec from polluting the global Rails.application state, which was causing an order-dependent spec failure.

I'm not sure if this still tests everything we want to, so that needs to be checked carefully.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer