"Fix DEV_MODE handling and RAGAgent initialization"

- Prioritize DEV_MODE explicitly during model selection
- Avoid ambiguous `model is None` checks
- Ensure model argument is always a valid identifier
- Clean up requirements.txt (remove duplicate entry, add newline at EOF)

This improves startup reliability and aligns with expected development
and production behavior.

Author: Naitik120gupta

requirements.txt

@@ -18,4 +18,4 @@ itsdangerous
 httpx
 pydantic-settings
 sentence-transformers
-python-dotenv
+python-dotenv