Update langchain packages to v0.2 for compatibility

- Upgrade langchain to 0.2.6 for langchain-core 0.2.x compatibility
- Upgrade langchain-community to 0.2.6
- Update langchain-core to 0.2.11 to satisfy langchain-openai>=0.2.2 requirement
- Resolve langchain-core version conflicts between packages

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

requirements.txt

@@ -6,10 +6,10 @@ numpy==1.24.3
 openpyxl==3.1.2
 
 # Fixed Langchain components
-langchain==0.1.20
-langchain-community==0.0.38
+langchain==0.2.6
+langchain-community==0.2.6
 langchain-openai==0.1.8
-langchain-core==0.1.52
+langchain-core==0.2.11
 sentence-transformers==2.2.2
 faiss-cpu==1.7.4
 transformers==4.36.0