Improve query rewrite for search context

[jtibshirani]

When gathering the initial search context for a repo, we use the 'fast' LLM to create a keyword search from the user's question. In evals, we found that this query rewrite made things worse for simple questions, so we only enabled it for longer or non-English questions.. I recently dug into this, and figured out this was because the rewrite prompt was not performing well.

This PR improves the rewrite prompt so we can remove the special-casing around short English questions. Benefits:

• Removes a "gotcha" that was making the mental model more complicated.
• It will let us switch to NLS search instead to power context search, which always expects keyword-style queries.
• The improved query rewrite is significantly faster. Here is the average rewrite time during the "Tool Call" eval:
  • Before: 1556.10ms
  • After: 677.25ms

Downside: single-sentence Chat questions with an @-repo mention become a bit slower, as they now rewrite the query.

FAQ: why are you working on this? We're reworking all of Chat!
This didn't take me much time at all. Also I need this to unblock my PR to move to Cody context to NLS search under-the-hood and eliminate the confusing duplication.

Test plan

Updated unit tests. I reran the "Tool Call" evals locally and checked that there is no regression. I was actually hoping for an improvement, but even without one this feels worth it.