feat(anthropic): support custom httpx client for mTLS and certificate-based authentication

### 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 - [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 I would like to add `http_client` and `http_async_client` optional fields to the `ChatAnthropic` class. These fields will allow users to provide their own pre-configured `httpx.Client` or `httpx.AsyncClient` instances, which will be passed directly to the underlying Anthropic SDK. ### Use Case I'm trying to build an application that requires **mTLS (mutual TLS)** or **certificate-based authentication** to connect to an Anthropic API proxy in a highly secure enterprise environment. Currently, I have to work around this by manually monkey-patching or wrapping the internal client creation, which is fragile. This feature would help users to easily configure security-related transport settings that are only accessible through a custom `httpx` client. ### Proposed Solution I think this could be implemented by adding two optional fields to `ChatAnthropic`: ```python http_client: Any = Field(default=None, exclude=True) http_async_client: Any = Field(default=None, exclude=True) ``` I…