An image attachment only got through if the model name was on a short
built-in list. Anything else was treated as text-only and the image was
quietly dropped, so the model never saw it. That left out a lot of the
smaller vision models you can run locally (moondream was the one I hit).
Pulled the check into is_vision_model() in chat_helpers, broadened it to
cover those, and added a test. Models that already worked are unaffected.
Fixes#124.
