Ollama not able to do structured outputs for gpt-oss (upstream issue)

### 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 read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example). - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Example Code The following code does not work ```python import dotenv from typing import Optional from pydantic import BaseModel, Field from rich.console import Console from langchain_ollama import ChatOllama from langchain_openai import ChatOpenAI # dotenv.load_dotenv() # llm = ChatOpenAI(model="gpt-4o-mini") # works llm = ChatOllama(model="gpt-oss:20b", reasoning=True) # does not work # llm = ChatOllama(model="llama3.1:8b") # works console = Console() class Joke(BaseModel): """Joke to tell user.""" setup: str = Field(description="The setup of the joke") punchline: str = Field(description="The punchline to the joke") rating: Optional[int] = Field( default=None, description="How funny the joke is, from 1 to 10" ) structured_llm = llm.with_structured_output(Joke, method="json_schema") response = structured_llm.invoke("Tell me a joke about cats") console.print(response) ``` ### Error Message and Stack Trace (if applicable) test_repo on  full_clean_refactor_langchain_switch [$!?] is 📦 v0.1.0 via  v22.20.0 via 🐍 v3.12.0 (test_repoenv) ❯ /home/test_user/Repos/test_repo/.venv/bin/python /home/test_user/Repos/test_repo/backend/services/ask_test_repo/src/langchain_structured_outputs_test.py Traceback (most recent call last): File "/home/test_user/Repos/t…