feat(anthropic): add http_client and http_async_client fields to ChatAnthropic (parity with ChatOpenAI)

### 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 ChatOpenAI already supports passing a custom httpx.Client and httpx.AsyncClient directly via http_client and http_async_client fields (lines 763–775 in chat_models/base.py). This is used in practice for SSL customization, custom proxies, and corporate network environments. ChatAnthropic has no equivalent. It always builds its own httpx client internally via _get_default_httpx_client(), and there is no way to inject a pre-configured one. The Anthropic SDK itself does support http_client as a constructor argument — ChatAnthropic just never exposes it. ### Use Case - Disable SSL verification in dev/test environments: httpx.Client(verify=False) - Use a corporate CA bundle: httpx.Client(verify="/etc/ssl/certs/corporate-ca.crt") - Custom transport layers, timeouts per-request, or mTLS certificates - Environments where global SSL settings cannot be changed ### Proposed Solution ```python import httpx from langchain_anthropic import ChatAnthropic # Custom SSL CA bundle llm = ChatAnt…