fix: stop logging a traceback twice per failed vector search - #29981

A vector DB outage renders a full traceback twice for every collection and query pair. query_doc and query_doc_with_hybrid_search each log and then re-raise, and the fan-out handler that catches them logs the same exception again, so twenty knowledge bases and five expanded queries turn one outage into hundreds of identical stack traces per message on every replica.

Both helpers lose the try/except that only logged before re-raising. Each fan-out keeps reporting failures through its return value and now carries the collection name back, so one record after the gather names every collection that failed and attaches a single traceback. The hybrid path logs before its existing raise, so a total failure is still reported before the caller falls back to vector search.

Measured with every collection failing, the vector fan-out drops from 1000 traceback records for fifty collections across ten queries to one, and the hybrid fan-out from 100 for ten collections across five queries to one. Results are byte-identical across eighteen and twenty-six scenarios covering healthy, failing, partial and degenerate inputs.

Three limits worth stating. Nine of the fifteen vector clients swallow the error inside search() and return None, chroma and pgvector among them, so those backends see neither the old tracebacks nor the new record. get_sources_from_items queries one attached item at a time, so a chat with twenty knowledge bases still logs twenty records. And with hybrid search on a total outage logs once from each fan-out as the caller falls back, down from two per pair plus one.

The old "All collection queries failed" warning goes with this. The replacement cannot express that case, because a collection can fail one query and succeed another, so naming every failed collection no longer implies an empty payload.