fix: Restore embedding-mode parameter to all examples

- All examples now have --embedding-mode parameter (unified interface benefit)
- Default is 'sentence-transformers' (consistent with original behavior)
- Users can now use OpenAI or MLX embeddings with any data source
- Maintains functional equivalence with original scripts

tests/test_document_rag.py

@@ -1,5 +1,5 @@
 """
-Test document_rag (formerly main_cli_example) functionality using pytest.
+Test document_rag functionality using pytest.
 """
 
 import os