tests: add _setup_image_processor to MiniCPMV4_6 with scale_resolution=64 and max_slice_nums=1
Default MiniCPMV4_6ImageProcessor uses scale_resolution=448 with max_slice_nums=9,
producing up to 21 MB pixel_values per image. Reducing to 64×1 cuts per-test RSS.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>