ChatMistralAI incompatible token output format for LangSmith traceability

### 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) - [ ] 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 import asyncio import json import os import random import time from decimal import Decimal from dotenv import load_dotenv from langchain_core.messages import HumanMessage, SystemMessage from langchain_mistralai.chat_models import ChatMistralAI from langsmith import Client, traceable load_dotenv() SYSTEM_PROMPT = """You are a knowledgeable travel planning assistant. Your role is to help users plan detailed itineraries for trips around the world. You understand geography, local customs, transportation options, seasonal weather patterns, and popular attractions. You can suggest accommodations ranging from budget hostels to luxury resorts, recommend local cuisine and restaurants, and help optimize travel routes for efficiency and enjoyment. You always consid…