[Agentic Search] Improve vector search UX

[ohltyler]

There's several helpful things that could be done to improve UX for users leveraging agentic search against knn indices, including:

• highlighting/labeling knn-enabled indices
• highlighting/labeling knn_vector fields for such indices
• improved presets (frontend overrides or backend documentation) to improve model capabilities in generating valid neural queries in the produced DSL for the query planning tool