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/sambanova/sambanova.py

@@ -19,9 +19,6 @@ class SambaNovaInferenceAdapter(OpenAIMixin):
     SambaNova Inference Adapter for Llama Stack.
     """
 
-    def get_api_key(self) -> str:
-        return self.config.api_key.get_secret_value() if self.config.api_key else ""
-
     def get_base_url(self) -> str:
         """
         Get the base URL for OpenAI mixin.