fix(rag): Resolve RAG test failures and race conditions

- Fixed circular import in chromadb by using lazy imports in 
ag_engine.py.
- Moved RAG engine initialization to background threads in AppController to avoid blocking UI.
- Added _rag_engine_lock to prevent race conditions during engine re-initialization.
- Updated Gemini embedding model to gemini-embedding-001 (available) from 	ext-embedding-004 (not found).
- Fixed _rebuild_rag_index to use fresh 
ag_engine instance from self in every iteration.
- Optimized 	est_rag_phase4_final_verify.py and 	est_rag_phase4_stress.py to wait for RAG sync before continuing.
- Added dummy embedding fallback in LocalEmbeddingProvider if sentence-transformers fails to load.

tests/test_rag_phase4_final_verify.py

@@ -34,11 +34,14 @@ def test_phase4_final_verify(live_gui):
   client.set_value('current_provider', 'gemini_cli')
   client.set_value('gcli_path', os.path.abspath(os.path.join(os.path.dirname(__file__), "mock_gcli.bat")))
   
-  # Wait for settings to apply
-  for _ in range(50):
-   if client.get_value('rag_emb_provider') == 'local':
+  # Wait for settings to apply and engine to sync
+  success = False
+  for _ in range(100):
+   if client.get_value('rag_emb_provider') == 'local' and client.get_value('rag_status') == 'ready':
+    success = True
     break
-   time.sleep(0.1)
+   time.sleep(0.5)
+  assert success, f"RAG sync failed. Status: {client.get_value('rag_status')}"
 
   # 3. Trigger Initial Indexing
   print("[VERIFY] Triggering indexing...")