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/gemini/gemini.py

@@ -17,8 +17,5 @@ class GeminiInferenceAdapter(OpenAIMixin):
         "text-embedding-004": {"embedding_dimension": 768, "context_length": 2048},
     }
 
-    def get_api_key(self) -> str:
-        return self.config.api_key or ""
-
     def get_base_url(self):
         return "https://generativelanguage.googleapis.com/v1beta/openai/"