added image resize config for MLX vision input

[RaajeevChandran]

Summary

enforce a fixed image resize config for MLX vision models to avoid fatal crash due to maximum allowed buffer size. fixes #634

Test Plan

1. Select an MLX vision model
2. Select a high resolution image and tap send
3. The app must run without crashing

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+

[tpae]

Would it resize all images to a square?

[RaajeevChandran]

Yes. That's intentional because I have the exact same config for one of my iOS apps that uses MLX. Using a square size config not only resolved the crash but also reduced the model response time. Afaik there won't be any issues for the models to process the image.

Attaching a few test cases here:

1. high res landscape image (6000x2848)

2. low res portrait image (960x1280)

[tpae]

I see. What about smaller images? will it upscale it?

[RaajeevChandran]

Yes, small images would get upscaled due to how MediaProcessing.apply in MLXVLM works since it doesn't clamp the scale to 1.0. The impact is negligible in practice but I've gone ahead and addressed it anyway by pre processing images before they reach the library. Small images now pass through at their native resolution while large images still get downscaled as intended

[tpae]

Looks great! Thank you for your contribution!