feat(anthropic): expose response headers in response_metadata

### Submission checklist - [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 - [x] 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 - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description Expose the HTTP response headers returned by Anthropic's API (request-id, anthropic-ratelimit-*, etc.) on AIMessage.response_metadata, behind an opt-in flag — mirroring the include_response_headers parameter already available on ChatOpenAI. Apply to both streaming and non-streaming paths (invoke, ainvoke, stream, astream). ### Use Case We log every LLM call in our production backend and want to store Anthropic's request-id (`req_011...`) alongside each call so we can jump from a log line straight to the Anthropic console when debugging. Same story for the rate-limit headers — we want to surface them to ops dashboards. Right now that header never reaches user code from ChatAnthropic, so we can't correlate application logs with the Anthropic console without resorting to hacks (see Alternatives). ### Proposed Solution Add an include_response_headers: bool = False option to ChatAnthropic, aligned with how it works in ChatOpenAI. When enabled, the response headers should be exposed …