feat(cart): enhance item update functionality to support variant changes and stock management; update related request validation and resource formatting; add tests for variant handling in cart

app/Domains/Cart/Requests/UpdateCartItemQuantityRequest.php

@@ -19,7 +19,7 @@ class UpdateCartItemQuantityRequest extends FormRequest
         return [
             'cantidad' => ['required', 'integer', 'min:1'],
             'catalog_item_id' => ['prohibited'],
-            'variant_id' => ['prohibited'],
+            'variant_id' => ['sometimes', 'nullable', 'integer'],
         ];
     }
 }