docs: update the code snippet for LangGraph Agent template

PiperOrigin-RevId: 716303416

Author: yeesian

vertexai/preview/reasoning_engines/templates/langgraph.py

@@ -260,23 +260,18 @@ def __init__(
  llm = ChatVertexAI(model_name=model, **model_kwargs)
 
  # runnable_builder
- from langchain import agents
- from langchain_core.runnables.history import RunnableWithMessageHistory
+ from langgraph.prebuilt import create_react_agent
  llm_with_tools = llm.bind_tools(tools=tools, **model_tool_kwargs)
- agent_executor = agents.AgentExecutor(
- agent=prompt | llm_with_tools | output_parser,
+ runnable = create_react_agent(
+ llm_with_tools,
  tools=tools,
- **agent_executor_kwargs,
- )
- runnable = RunnableWithMessageHistory(
- runnable=agent_executor,
- get_session_history=chat_history,
  **runnable_kwargs,
  )
  ```
 
- By default, no checkpointer is used. To enable checkpointing, provide a
- `checkpointer_builder` function that returns a checkpointer instance.
+ By default, no checkpointer is used (i.e. there is no state history). To
+ enable checkpointing, provide a `checkpointer_builder` function that
+ returns a checkpointer instance.
 
  **Example using Spanner:**
  ```python