feat(ticket-selector): refactor variant handling to use single variant property and update related tests

src/app/core/services/cart/cart.interface.ts

@@ -1,7 +1,7 @@
 export interface CartItemProduct {
   nombre: string;
   imagen: string | null;
-  variants?: CartItemVariant[];
+  variant: CartItemVariant | null;
 }
 
 export interface CartItemVariantOption {

src/app/core/services/cart/cart.service.spec.ts

@@ -27,6 +27,12 @@ describe('CartService', () => {
         product: {
           nombre: 'Test Product (Size: M)',
           imagen: null,
+          variant: {
+            id: 10,
+            precio: '10.00',
+            stock_tecnico: 1,
+            values: { size: 'M' },
+          },
         },
       },
     ],