feat: use SecretStr for inference provider auth credentials (#3724)

# What does this PR do?

use SecretStr for OpenAIMixin providers

- 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)

addresses #3517 

## Test Plan

ci w/ new tests

llama_stack/providers/remote/inference/vertexai/config.py

@@ -6,7 +6,7 @@
 
 from typing import Any
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, SecretStr
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
@@ -25,6 +25,8 @@ class VertexAIProviderDataValidator(BaseModel):
 
 @json_schema_type
 class VertexAIConfig(RemoteInferenceProviderConfig):
+    auth_credential: SecretStr | None = Field(default=None, exclude=True)
+
     project: str = Field(
         description="Google Cloud project ID for Vertex AI",
     )