has_provider_credentials

Check if credentials are available for a provider.

Resolution order:

1. Config-file providers (config.toml) with api_key_env — takes priority so user overrides are respected.
2. Config-file providers with class_path but no api_key_env — assumed to manage their own auth (e.g., custom headers, JWT, mTLS).
3. Hardcoded PROVIDER_API_KEY_ENV mapping (anthropic, openai, etc.).
4. For any other provider (e.g., third-party langchain provider packages), credential status is unknown — the provider itself will report auth failures at model-creation time.