feat: use SecretStr for inference provider auth credentials

- RemoteInferenceProviderConfig now has auth_credential: SecretStr
- the default alias is api_key (most common name)
- some providers override to use api_token (RunPod, vLLM, Databricks)
- some providers exclude it (Ollama, TGI, Vertex AI)

llama_stack/providers/remote/inference/anthropic/anthropic.py

@@ -29,9 +29,6 @@ class AnthropicInferenceAdapter(OpenAIMixin):
     #     "voyage-multimodal-3": {"embedding_dimension": 1024, "context_length": 32000},
     # }
 
-    def get_api_key(self) -> str:
-        return self.config.api_key or ""
-
     def get_base_url(self):
         return "https://api.anthropic.com/v1"