Previously MultiQuery only accepted vector sub-queries by using
get_vector_field() to look up each sub-query's field. FTS sub-queries
(whose field_name points to an FTS-indexed string column) would fail
with "Vector field not found".
Changes:
- collection.cc: use get_field() uniformly in MultiQuery path; let
validate_and_sanitize() check type compatibility internally, which
is consistent with the single-query path.
- query_executor.py: allow SingleVectorQueryExecutor to accept
multi-query when it contains an FTS query (with reranker), and route
to C++ MultiQuery fast path.
- Add test_collection_fts_vector_hybrid.py covering hybrid retrieval
ranking, scoring, filter, validation, and edge cases.
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