Add three new test cases to test_vision_api.py that address the TODO
for testing with multiple images and no images:
- test_vision_chat_completion_multiple_images: verifies the server
handles multiple image_url content parts in a single request
- test_vision_chat_completion_no_image: verifies text-only messages
work correctly on a multimodal model
- test_vision_chat_completion_no_image_content_parts: verifies
content parts with only text type (no image_url) work correctly
The audio test TODO is narrowed to note it needs a model with audio
input support, which the current tinygemma3 test model lacks.
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