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/azure/config.py

@@ -32,9 +32,6 @@ class AzureProviderDataValidator(BaseModel):
 
 @json_schema_type
 class AzureConfig(RemoteInferenceProviderConfig):
-    api_key: SecretStr = Field(
-        description="Azure API key for Azure",
-    )
     api_base: HttpUrl = Field(
         description="Azure API base for Azure (e.g., https://your-resource-name.openai.azure.com)",
     )