support model_armor with disabling vertexai

I would like to pass model_armor_config to the Generative Language API call, and it appears that the code below is currently the only way to invoke the API while including model_armor_config. Is there a way to make this work with the vertexai=False flag, to ensure that the request is sent to the Gemini API platform rather than Vertex AI?
```
client_options = {"api_endpoint": "us-east4-aiplatform.googleapis.com"}
client = aiplatform_v1beta1.PredictionServiceClient(client_options=client_options)

request = aiplatform_v1beta1.GenerateContentRequest(
model="gemini-2.0-flash-001",
contents=[content.Content(role="user", parts=[content.Part(text="Hello world")])],
model_armor_config={
"prompt_template_name": f"projects/{PROJECT_ID}/locations/{LOCATION}/templates/{TEMPLATE_ID}"
}
)
```

Start by tracing the vertexai=False path and the Generative Language API call in the Python client, then compare it with PredictionServiceClient and GenerateContentRequest. Verify whether model_armor_config can be passed through while targeting the Gemini API platform, and confirm the request reaches the intended platform when the change is complete.

Written by the indexing model from the issue text.