Missing is_bundle for product bundle

When selling a kit or bundle (different products packaged together), is_bundle must be TRUE. Otherwise Google may flag it as misrepresented.

Bundles often have unique SKUs but lack the is_bundle declaration, making them indistinguishable from regular products to Google.

1. Set is_bundle=TRUE for any product that's a curated kit or assembly of different items.
2. Use the GTIN of the bundle if one exists; otherwise apply identifier_exists=FALSE plus brand+mpn.
3. List the bundle's main product first in title and description.

• Multipacks getting is_bundle=TRUE — they need multipack instead