ProviderStrategy does not allow passing strict into the mistral ai JSON Schema response_format

### 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 - [ ] 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 - [x] 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 dotenv import load_dotenv from pydantic import BaseModel, Field from typing import Literal from langchain.agents import create_agent from langchain.agents.structured_output import ProviderStrategy load_dotenv() class MeetingAction(BaseModel): """Action items extracted from a meeting transcript.""" task: str = Field(description="The specific task to be completed") assignee: str = Field(description="Person responsible for the task") priority: Literal["low", "medium", "high"] = Field(description="Priority level") agent = create_agent("mistral-medium-3-5",response_format=ProviderStrategy(MeetingAction)) res = agent.invoke({ "messages": [{"role": "user", "content": "From our meeting: Sarah needs to update the project timeline as soon …