Interactions API: Submitting a multimodal request routes to model that doesn't support multimodal requests

When submitting a multimodal request with the background parameter the request appears to be routed internally to a model with no multimodal support.

#### Environment details

- Programming language: Python
- OS: Mac
- Language runtime version: 3.12
- Package version: 2.20.0

#### Steps to reproduce

1. Craft a multimodal request that includes the background parameter

```python
uploaded_file = client.files.upload(file="loom.webp")

interaction = client.interactions.create(
model="gemini-3.7-flash",
input=[
{
"type": "text",
"text": "What is in this image?",
},
{
"type": "image",
"uri": uploaded_file.uri,
"mime_type": uploaded_file.mime_type
},
],
background=True,
)
```

Error text:
```
Traceback (most recent call last):
File "/Users/graeme/Code/sandbox/.venv/lib/python3.13/site-packages/google/genai/_gaos/interactions.py", line 711, in create
return _speakeasy_parse_response(http_res)
File "/Users/graeme/Code/sandbox/.venv/lib/python3.13/site-packages/google/genai/_gaos/interactions.py", line 489, in _speakeasy_parse_response
raise errors.CreateInteractionClientError(
response_data, http_res, http_res_text
)
google.genai._gaos.errors.createinteraction.CreateInteractionClientError: Image input modality is not enabled for models/gemini-3.7-flash-agent
```

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!