Documents the 5-phase investigation, root cause analysis (type contract
mismatch between _rag_search_result's declared return type
Result[list[Metadata]] and actual return List[RAGChunk]), the surgical
production + test fixes, verification (5/5 consecutive PASS runs of
the fixed test, 25/26 RAG tests pass), and lessons learned about
silent exceptions in worker threads.
Also notes one pre-existing regression (test_rag_collection_dim_mismatch_recreates_collection)
from commit 24e93a75 that is out of scope for this fix.
ed
4b86f87e3b