deps: add langchain-deepseek for native DeepSeek support

Adds official LangChain DeepSeek integration to replace ChatOpenAI
wrapper approach for DeepSeek models. Native integration provides:
- Better tool_calls argument parsing
- DeepSeek-specific error handling
- No OpenAI compatibility layer issues

Version 0.1.20+ includes tool calling support for deepseek-chat.

requirements.txt

@@ -1,6 +1,7 @@
 langchain==1.0.2
 langchain-openai==1.0.1
 langchain-mcp-adapters>=0.1.0
+langchain-deepseek>=0.1.20
 fastmcp==2.12.5
 fastapi>=0.120.0
 uvicorn[standard]>=0.27.0