ChatOpenAI.with_structured_output(..., method="json_schema").stream() drops response headers / x-request-id in response_metadata

### Submission checklist - [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 - [x] 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-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python from pathlib import Path from typing import Any from langchain_core.callbacks import BaseCallbackHandler from langchain_openai import ChatOpenAI from pydantic import BaseModel class SimpleAnswer(BaseModel): answer: str class HeaderCaptureHandler(BaseCallbackHandler): """Records whether ``x-request-id`` arrived via on_llm_end / on_llm_new_token.""" def __init__(self) -> None: self.x_request_id: str | None = None def _grab(self, message: Any) -> None: if message is None: return headers = (getattr(message, 'response_metadata', None) or {}).get('headers') or {} rid = headers.get('x-request-id') or headers.get('X-Request-Id') if rid and self.x_request_id is None: self.x_request_id…