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/ollama/ollama.py

@@ -59,7 +59,7 @@ class OllamaInferenceAdapter(OpenAIMixin):
         return self._clients[loop]
 
     def get_api_key(self):
-        return "NO_KEY"
+        return "NO KEY REQUIRED"
 
     def get_base_url(self):
         return self.config.url.rstrip("/") + "/v1"