Inconsistent/confusing `astream` returned `AsyncIterator`

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] 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 ### Reproduction Steps / Example Code (Python) ```python async for stream_mode, data in self._agent.astream( # The output of each step in the graph. The output shape depends on the stream_mode. {"messages": [{"role": "user", "content": message}]}, stream_mode = modes, # Use this to stream all values in the state after each step. config = config, # This is needed by Checkpointer subgraphs = subgraphs ): logging.debug(f"stream_mode: {stream_mode}") if stream_mode == "values": messages.append(data["messages"][-1]) data["messages"][-1].pretty_print() elif stream_mode == "updates": for source, update in data.items(): if source in ("model", "tools"): update["messages"][-1].pretty_print() await output_queue.put(update["messages"][-1]) ``` ### Error Message a…