has_provider_credentials

Check if credentials are available for a provider.

Checks in order:

1. Config-file providers (config.toml) — takes priority so user overrides (e.g., custom api_key_env or base_url) are respected.
2. Hardcoded PROVIDER_API_KEY_ENV mapping (anthropic, openai, etc.).
3. Langchain's _SUPPORTED_PROVIDERS registry — if the provider is known to init_chat_model, credential status is unknown; the provider itself will report auth failures at model-creation time.