`ChatAnthropic`: support a callable / token-provider for `api_key` (auto-refreshing credentials, e.g. Azure Entra ID) like `ChatOpenAI` does

### 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 `ChatOpenAI` accepts `api_key` as either a static secret **or a sync/async callable that returns a string**: ```python openai_api_key: ( SecretStr | None | Callable[[], str] | Callable[[], Awaitable[str]] ) = Field(alias="api_key", ...) ``` This lets the key be resolved fresh on every request, which is exactly what short-lived bearer tokens need: ```python credential = DefaultAzureCredential() ChatOpenAI( model=model, api_key=lambda: credential.get_token(scope).token, # re-invoked per call base_url=base_url, ) ``` `ChatAnthropic` only accepts a **static** value: ```python anthropic_api_key: SecretStr = Field( alias="api_key", default_factory=secret_from_env("ANTHROPIC_API_KEY", default=""), ) ``` **Request:** allow `ChatAnthropic.api_key` to accept a sync/async `Callable[[], str]` (and/or add a dedicated `api_key_provider` / `auth_token_provider` parameter), so the credential is re-resolved on each request instead of being frozen at construction time. T…