Structured Output Fails When Appending Assistant Response to Context

### Submission checklist - [x] This is a bug, not a usage question. - [ ] 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. - [ ] 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 - [ ] 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 pydantic import BaseModel from langchain_openai import ChatOpenAI class Step(BaseModel): explanation: str output: str class MathReasoning(BaseModel): steps: list[Step] final_answer: str llm = ChatOpenAI(model="openai/gpt-oss-20b", base_url=BASE_URL, api_key=API_KEY) structured_llm = llm.with_structured_output(MathReasoning) in_list = [ {"role": "system", "content": "You are a helpful math tutor."}, {"role": "user", "content": "how can I solve 8x + 7 = -23"}, ] # First call — works response = structured_llm.invoke(in_list) # Append assistant response as context in_list.append({"role": "assistant", "content": MathReasoning.model_dump_json(response)}) in_list.append({"role": "user", "content": "what is the final answer?"}) # Sec…