* feat: support timescale pg_textsearch as text search extension
* refactor: deduplicate text search query in retrieve_semantic_bm25_combined
Instead of maintaining 3 complete query copies (native, vchord, pg_textsearch),
now we:
- Build backend-specific parts (score_expr, order_by, where_filter)
- Use a single query template with injected backend-specific parts
This makes maintenance easier - changes to the semantic CTE or overall structure
only need to be made once.
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