Closes#32320
This PR updates the `langgraph_agentic_rag.ipynb` notebook to clarify
that LangGraph does not automatically prepend a `SystemMessage`. A
markdown note and an inline Python comment have been added to guide
users to explicitly include a `SystemMessage` when needed.
This improves documentation for developers working with LangGraph-based
agents and avoids confusion about system-level behavior not being
applied.
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Co-authored-by: Mason Daugherty <mason@langchain.dev>