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/azure.py

@@ -16,9 +16,6 @@ class AzureInferenceAdapter(OpenAIMixin):
 
     provider_data_api_key_field: str = "azure_api_key"
 
-    def get_api_key(self) -> str:
-        return self.config.api_key.get_secret_value()
-
     def get_base_url(self) -> str:
         """
         Get the Azure API base URL.

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)",
     )