feat: implement prefetching for AI settings search

[rzhao271]

This PR changes the behaviour of AI settings search in the Settings editor in the following ways

• The Settings editor will only search and show results from the more performant algorithms, i.e. string-matching and TF-IDF. Previously, it was fetching embeddings results as well for users with Copilot enabled.
• The Settings editor will start an AI search in the background after those searches are complete. The search uses free models and will only apply to users with Copilot enabled.
• The sparkle is now hidden until there are AI search results available. Clicking on the toggle now immediately flips the Settings editor search results between the non-AI ones and the AI ones.