Add get_provider_models() API for listing models from each provider
Add list_models() to anthropic, openai, and google provider modules, each calling the SDK client.models.list() and returning raw dicts via model_dump(). Add get_provider_models() in think.providers.__init__ as a dispatcher to the selected provider module. Add unit tests covering anthropic/openai/google dispatch, unknown provider ValueError, and __all__ exports.