docs: clarify SystemMessage usage in LangGraph agent notebook (#32320) (#32346)

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.

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

cookbook/langgraph_agentic_rag.ipynb

@@ -79,6 +79,17 @@
     "tool_executor = ToolExecutor(tools)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "168152fc",
+   "metadata": {},
+   "source": [
+    "📘 **Note on `SystemMessage` usage with LangGraph-based agents**\n",
+    "\n",
+    "When constructing the `messages` list for an agent, you *must* manually include any `SystemMessage`s.\n",
+    "Unlike some agent executors in LangChain that set a default, LangGraph requires explicit inclusion."
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "fe6e8f78-1ef7-42ad-b2bf-835ed5850553",