Ensure group acts as a namespace for polymorphic routes

What does this MR do?

It fixes a recent change (!8604 (merged)), not yet in the stable branch, btw.

Are there points in the code the reviewer needs to double check?

There were already specs but the project was in a personal namespace rather than a group, so the tests would pass.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26810 (closed)