Update multiple products in a batch as a vendor.

Image URLs are appended only when their trimmed source URL is not already
stored for the product. Duplicate URLs in the request are ignored. Existing
images retain their ordering and mirrored local files; images_added counts
only newly inserted images.

With source: bulk or source: shopify, each product may set sync_images: true
and supply its complete images URL array. This retains matching images, removes
missing URLs and duplicate stored rows, and applies array order with the first
image primary. An empty array clears the gallery; omitting images leaves it
unchanged unless explicit removal options are used. Successful reconciliation
returns images_synced: true. Do not combine it with explicit removal options.