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/watsonx/config.py

@@ -7,7 +7,7 @@
 import os
 from typing import Any
 
-from pydantic import BaseModel, Field, SecretStr
+from pydantic import BaseModel, Field
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
@@ -25,10 +25,6 @@ class WatsonXConfig(RemoteInferenceProviderConfig):
         default_factory=lambda: os.getenv("WATSONX_BASE_URL", "https://us-south.ml.cloud.ibm.com"),
         description="A base url for accessing the watsonx.ai",
     )
-    api_key: SecretStr | None = Field(
-        default_factory=lambda: os.getenv("WATSONX_API_KEY"),
-        description="The watsonx API key",
-    )
     project_id: str | None = Field(
         default_factory=lambda: os.getenv("WATSONX_PROJECT_ID"),
         description="The Project ID key",