Settings editor appears slow when it falls back to AI search

[bpasero]

For a user its not clear if a search for a setting is just slow or done by AI:

We need to better communicate to the user that some fallback search was used, otherwise it seems VS Code has an incredibly slow settings search.

//cc @kkbrooks

[rzhao271]

I believe I have worked around this issue with the new implementation of AI settings search, ref #251840.

The search UX should now be similar to the search UX for users without Copilot.