fix: avoid double resize when fetch_video receives image list

[HT-Yuan]

fix: avoid double resize when fetch_video receives image list

What's changed

Extract load_image from fetch_image() to handle image loading and RGB conversion without resizing.
In fetch_video's image list branch, use load_image() instead of fetch_image() so that images are only resized once (at VIDEO level with dynamic max_pixels cap).

What's fixed

Avoid the previous double resize that caused:

1. Unnecessary computation overhead
2. Image quality loss

Fixes #1833

[HT-Yuan]

@ShuaiBai623 Can we merge this MR? We believe it will help improve the inference speed of Qwen.