Fetch a specific set of products by product_id and/or variant_id,
returning the same payload shape as Full catalogue filtered to the
IDs you asked for. Designed to be called in response to a
CatalogueUpdated webhook so you
only refetch the rows that changed instead of the entire catalogue.

• At least one of product_ids or variant_ids must be non-empty.
• The combined list cap is 500 IDs per request.
• Matched products are returned in full (all of their variants),
  not just the matched variant — so a single response is enough to
  rebuild your local cache row.
• IDs that don't resolve to anything currently associated with the
  location are silently omitted; treat their absence as "deleted from
  this storefront" and remove them from your cache.
• Stock control still applies — out_of_stock is per online location,
  consistent with Full catalogue.