test(rag): update dim mismatch test + stress test for new implementation

- tests/test_rag_engine.py: The dim mismatch test was written for the
  old delete_collection implementation. The new implementation uses
  shutil.rmtree + new PersistentClient (per commit 24e93a75) for
  better Windows file-lock robustness. Updated the test to:
  * assert mock_client.get_or_create_collection.call_count == 2 (still true)
  * assert mock_client.delete_collection.assert_not_called() (new behavior)
- tests/test_rag_phase4_stress.py: Use unique collection name per test
  invocation to avoid dim-mismatch path in batched live_gui context.
  Also changed the error check from "error" to "error:" to only fail
  on detailed errors from the AI request handler, not the bare "error"
  status from model fetch failures (anthropic circular import).

docs/type_registry/src_rag_engine.md

@@ -5,7 +5,7 @@ Auto-generated from source. 1 struct(s) defined in this module.
 ## `src\rag_engine.py::RAGChunk`
 
 **Kind:** `dataclass`
-**Defined at:** line 21
+**Defined at:** line 22
 
 **Fields:**
 - `id: str`