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/nvidia/config.py

@@ -7,7 +7,7 @@
 import os
 from typing import Any
 
-from pydantic import Field, SecretStr
+from pydantic import Field
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
@@ -40,10 +40,6 @@ class NVIDIAConfig(RemoteInferenceProviderConfig):
         default_factory=lambda: os.getenv("NVIDIA_BASE_URL", "https://integrate.api.nvidia.com"),
         description="A base url for accessing the NVIDIA NIM",
     )
-    api_key: SecretStr | None = Field(
-        default_factory=lambda: SecretStr(os.getenv("NVIDIA_API_KEY")),
-        description="The NVIDIA API key, only needed of using the hosted service",
-    )
     timeout: int = Field(
         default=60,
         description="Timeout for the HTTP requests",