Streaming Output for Multi-Agent Systems

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description When developing using the framework at https://docs.langchain.com/oss/python/langchain/multi-agent/router-knowledge-base, I found that our internal call to the LangChain agent (mainly for quickly creating agents using LangChain's agent construction capabilities) is implemented as: ``` def query_notion(state: AgentInput) -> dict: """Query the Notion agent.""" result = notion_agent.invoke({ "messages": [{"role": "user", "content": state["query"]}] }) return {"results": [{"source": "notion", "result": result["messages"][-1].content}]} ``` This itself is just a function; as a result, it is only a graph_node_start node rather than an agent node, which leads to the inability to access the internal tool-related calls during streaming output. LangGraph's astream() cannot see the streaming events from the Agent that are called inside the node function. ### Use Case I encountered this situation while developing AI front-end and back-end applic…