When using OpenAI Responses API in streaming mode, there is no chunk from langchain that contains the entire function_call definition including arguments.

### 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) - [ ] langchain - [x] 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 ### Example Code (Python) ```python #!/usr/bin/env python3 """ Minimal reproducible example demonstrating that langchain_openai does not return tool_calls in the response.completed chunk. Issue: When streaming with responses API and tool calls, the response.completed chunk contains metadata (usage, response_metadata, parsed) but the tool_calls from the aggregated message are missing. Root cause (see base.py:4307-4324): - When chunk.type == "response.completed", the code constructs a full message using _construct_lc_result_from_responses_api(chunk.response) - It extracts parsed, usage_metadata, and response_metadata from that message - BUT it never extracts tool_calls or tool_call_chunks from the constructed message - The tool_call_chunks list is only built during streaming chunks, not from the completed response's aggregated…