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/runpod/runpod.py

@@ -24,10 +24,6 @@ class RunpodInferenceAdapter(OpenAIMixin):
 
     config: RunpodImplConfig
 
-    def get_api_key(self) -> str:
-        """Get API key for OpenAI client."""
-        return self.config.api_token
-
     def get_base_url(self) -> str:
         """Get base URL for OpenAI client."""
         return self.config.url