revert: ref_img_resize back to keep_ref_img_size BOOLEAN

The ref_img_resize string ("720"/"1024"/"origin") path on the C library
side has not shipped, so revert the Python side to pass the original
keep_ref_img_size bool to image_to_image() — matching commit 2ac0bf3.

ImageList exposes keep_ref_img_size BOOLEAN (default False). True keeps
the reference image at its original size (slower, higher quality);
False resizes the long side to 720 px for faster edits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>