openrouter json_schema is not enforced, llm doesn't know about the schema

### 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 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 - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [x] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs _No response_ ### Reproduction Steps / Example Code (Python) ```python import os from langchain_openrouter import ChatOpenRouter from pydantic import BaseModel, ConfigDict, Field os.environ["OPENROUTER_API_KEY"] = "x" schema = { "title": "ProductInfo", "description": "Structured product data extracted from user text", "type": "object", "properties": { "name": { "type": "string", "description": "Product name" }, "price_pln": { "type": "number", "description": "Price in PLN" }, "color": { "type": "string", "description": "Product color" }, "volume_ml": { "type": "integer", "description": "Volume in milliliters" }, "dishwasher_safe": { …