improve error handling for overly long search queries #2

when users submit search queries exceeding turbopuffer's 1024 character limit for BM25 text search, the application now returns a 400 Bad Request with a helpful error message instead of a generic 500 Internal Server Error.

changes:
- add TurbopufferError enum to categorize different error types
- parse turbopuffer API error responses to detect query length violations
- return 400 status with user-friendly message for query length errors
- maintain 500 status for genuine server errors

this fix ensures users understand the limitation and can adjust their queries accordingly, without falsely suggesting a server-side problem.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

the backend was correctly returning helpful error messages (like "search query is too long..."), but the frontend was only showing "search failed: Bad Request" because it only read response.statusText and never read the response body.

now when an API error occurs, the frontend:
1. reads the response body text
2. displays the actual error message from the backend
3. falls back to statusText if body reading fails

this means users will now see the full helpful message: "search query is too long (max 1024 characters for text search). try a shorter query."

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>