Support Anthropic Claude automatic caching in ChatOpenRouter

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### 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 ### Feature Description We would need the ability to enable Anthropic Claude automatic caching (at request top-level) using the openrouter integration (`ChatOpenRouter`). ### Use Case We need that for costs optimization when using Claude with OpenRouter. Automatic caching is recommended for long conversations. ### Proposed Solution _No response_ ### Alternatives Considered I tried this: ```python ChatOpenRouter( model="anthropic/claude-haiku-4.5", model_kwargs={"cache_control":{"type": "ephemeral"}} ) ``` But then got: ``` TypeError: Chat.send() got an unexpected keyword argument 'cache_control' ``` There might be a limitation in the underlying ˋopenrouterˋ library. ### Additional Context https://openrouter.ai/docs/guides/best-practices/prompt-caching#anthropic-claude > Automatic caching: Add a single cache_control field at the top level of your request. The system automatically applies the cache breakpoint to the last cacheable block and advances it forward as conversations grow. Best for …