ChatAnthropic cannot pass credentials/auth_token/prebuilt client to the anthropic SDK

### 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 `anthropic.Client` accepts three authentication inputs beyond `api_key`: `credentials=` (e.g. `WorkloadIdentityCredentials`), `auth_token=`, and a caller-built `http_client`. `ChatAnthropic` exposes none of them. `_client_params` builds `{"api_key": self.anthropic_api_key.get_secret_value(), ...}` and `_client`/`_async_client` construct the SDK client from that dict alone (langchain-anthropic 1.7.0, current master). The request: a public way to hand `ChatAnthropic` either the SDK's credential objects or a prebuilt client, so authentication modes the anthropic SDK already supports become reachable from LangChain. ### Use Case Keyless deployments. With workload identity federation, an identity broker mints short-lived tokens and the anthropic SDK exchanges them for Anthropic access tokens; no API key exists anywhere in the deployment. The SDK handles the whole flow through `credentials=`, but `ChatAnthropic` cannot pass it, so agents built on LangChain don't allow for keyless set…