Replaces self.client.delete_collection(name) with shutil.rmtree on the
collection directory + recreate PersistentClient. This is more robust
to file locks (WinError 32 on Windows) where the live_gui subprocess
holds the file lock on the chroma collection.
The original delete_collection call fails on locked files, leaving the
collection in a broken state (dim mismatch) that causes subsequent
RAG searches to hang. shutil.rmtree with ignore_errors=True handles
this case more gracefully.
Note: This fix is an improvement but may not fully resolve the
test_rag_phase4_final_verify timeout in batched runs. The fundamental
issue is that the live_gui subprocess (session-scoped fixture) holds
file locks on the workspace's .slop_cache, and the test's pre-test
cleanup cannot remove locked files from the same process. A complete
fix would require either changing the fixture scope or implementing
a more sophisticated lock-handling strategy in the RAG engine.
Diagnosis documented in docs/reports/DIAGNOSIS_test_rag_phase4_final_verify.md.
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