Fix agentic chat reasoning example won't start

Author: maxkorp

typescript-sdk/integrations/langgraph/examples/python/agents/agentic_chat_reasoning/agent.py

@@ -3,6 +3,8 @@
 """
 
 from typing import List, Any, Optional
+import os
+
 from langchain_core.runnables import RunnableConfig
 from langchain_core.messages import SystemMessage
 from langchain_openai import ChatOpenAI