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/openai/config.py

@@ -21,10 +21,6 @@ class OpenAIProviderDataValidator(BaseModel):
 
 @json_schema_type
 class OpenAIConfig(RemoteInferenceProviderConfig):
-    api_key: str | None = Field(
-        default=None,
-        description="API key for OpenAI models",
-    )
     base_url: str = Field(
         default="https://api.openai.com/v1",
         description="Base URL for OpenAI API",