OpenRouter / LiteLLM / OpenAI API compatibility across providers / proxies

### Privileged issue - [x] I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here. ### Issue Content Tracking for upcoming OpenRouter integration. It is not practical for `ChatOpenAI` to handle each addition that third party providers make to the Chat Completions standard. ## Related issues ### OpenRouter #31325 - User is attempting to access OpenRouter via `ChatAnthropic` - Recommended to use `ChatOpenAI` with warning. #32967 - User is attempting to access OpenRouter via `ChatOpenAI` - Structured output via `create_agent` fails because OpenRouter doesn't support the OpenAI SDK's `beta.chat.completions.parse` method #32977 - User is attempting to access OpenRouter via `ChatOpenAI` - `with_structured_output` fails with `JSONDecodeError` when OpenRouter returns malformed/incomplete JSON. Even with `include_raw=True`, the raw response is inaccessible on error. #32981 - User is attempting to access OpenRouter via `ChatOpenAI` - Reasoning content is not being captured by `ChatOpenAI` - Commenter states `ChatDeepSeek` is also not working #33643 - User requests native LiteLLM and OpenRouter integrations in LangChain. #33757 - User wants to use OpenRouter via `ChatOpenAI` to support `cache_control` (e.g. for Anthropic models) #34056 - User is accessing `gemini-3-pro-preview` with OpenRouter via `ChatOpenAI` but thought signatures are not being handled - Commenters recommended using `langchain-litellm` as a workaround but it is unclear if this is just compatibility by luck #34797 - User is attempting to access OpenRouter via `ChatOpenAI` - `ProviderStrategy` is not working #34962 - User requesting dedicated `langchain-openrouter` / `ChatOpenRouter` that does not inherit from `BaseChatOpenAI` because - Response schema and header contract differ - OpenRouter is a distinct provider, not an OpenAI variant - Proper tracing requires correct `_llm_type` identification - OpenRouter metadata (routing info) needs first-class supp…