Storefront API item catalog endpoints

The public catalog now uses item endpoints that return products and bundle price options together.

Storefront API v3 now uses GET /v3/stores/{store_id}/public/items and
GET /v3/stores/{store_id}/public/items/count for the public catalog feed
because the feed returns both products and bundle price options. Product detail remains
GET /v3/stores/{store_id}/public/products/{slug}, and bundle price option detail is
GET /v3/stores/{store_id}/public/bundle-price-options/{slug}. Catalog cards use
entity_type to distinguish product from bundle_price_option, while product
item_type keeps its product meaning such as physical or digital. Public cart item
and direct checkout item payloads use { "type": "bundle_price_option", "bundle_price_option_id": ... }
for bundle price options.