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/config.py

@@ -6,12 +6,16 @@
 
 from typing import Any
 
+from pydantic import Field, SecretStr
+
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 
 DEFAULT_OLLAMA_URL = "http://localhost:11434"
 
 
 class OllamaImplConfig(RemoteInferenceProviderConfig):
+    auth_credential: SecretStr | None = Field(default=None, exclude=True)
+
     url: str = DEFAULT_OLLAMA_URL
 
     @classmethod