Product query contract

Shared by CAROUSEL, GRID, PRODUCTCARD, FLASHSALE and MIXEDCAROUSEL's item_type: "products" page. Each of these widgets converts its params into a ProductQuery via ProductQuery.fromParams, then calls WidgetCallbacks.fetchProducts(query) to get the products to render.

Two equivalent shapes are accepted:

A type + id pair, resolved the same way as the action contract:

{ "type": "category", "id": 75 }

type	Resolves to
category / main_category	categoryId
collection	collectionId
brand / brands	brandId
group	groupId
filter	filter (uses params.filter, or id if that's absent)
search	search (uses params.search_text), or collectionId = id if absent
(other)	categoryId

Or direct filter fields, read as-is when there's no type/id:

{
  "category_id": 75,
  "collection_id": null,
  "brand_id": null,
  "group_id": null,
  "search": "OKUL",
  "search_fields": "title,subtitle",
  "order_by": "id|DESC",
  "limit": 10,
  "page": 1,
  "filter_name": "optional label",
  "except_product_ids": [1, 2, 3],
  "filter": "optional raw filter string",
  "link": "optional",
  "is_favorited_list": false,
  "is_bundle_product": false
}

order_by is a raw "field|DIRECTION" string (e.g. "id|DESC"), forwarded to your backend unparsed.

fetchProducts must return a List<ProductCardData> — see product-card-data.md for that shape.