mtmd : fix Pixtral OOM with large images by capping image_size to 1024 (#14326)

mtmd : fix Pixtral OOM with large images by capping image_size to 1024 (#14326)

Mistral Small 2506 models using Pixtral vision encoder were running out
of GPU memory when processing images larger than 1024x1024 pixels due to
exponential memory growth from unlimited image size.

This fix applies the same 1024x1024 limit used by Qwen2VL models to
prevent OOM issues while maintaining compatibility with existing models.