Tighten user notification API response types

[davelopez]

Part of #18532

This will exclude "broadcast" as a possible user notification response content as /api/notifications/* will never return a broadcast notification since we use /api/notifications/broadcast/* for those.

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[davelopez]

All test failures are unrelated