Fix missing imports in the multi-agent-collaboration example

examples/multi_agent/multi-agent-collaboration.ipynb

@@ -91,7 +91,8 @@
     "    FunctionMessage,\n",
     "    HumanMessage,\n",
     ")\n",
-    "\n",
+    "from langchain.tools.render import format_tool_to_openai_function\n",
+    "from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder\n",
     "from langgraph.graph import END, StateGraph\n",
     "from langgraph.prebuilt.tool_executor import ToolExecutor, ToolInvocation\n",
     "\n",