fix: resolve CI test failures (carts, stripe, shipping, scaffold, cart session)

- carts.test: add required product fields (parentCategorySlug, childCategorySlug)
  and variant fields (weight, weightUnit)
- stripeActions.test: use price in cents (2499) for variant/cart and expect
  unit_amount: 2499 in line_items assertion
- useShippingRate.test: expect fallback error message for plain Error rejections
- scaffold.test: enable @ alias in root vitest.config for storefront imports
- useCartSession.test: mock useConvexAuth instead of ConvexProviderWithClerk
  for reliable unit tests

Made-with: Cursor

convex/stripeActions.test.ts

@@ -129,7 +129,7 @@ async function setupFullCheckoutContext(
       productId,
       name: "1kg Bag",
       sku: "PK-001",
-      price: overrides?.price ?? 24.99,
+      price: overrides?.price ?? 2499,
       stockQuantity: overrides?.stockQuantity ?? 50,
       isActive: overrides?.isActive ?? true,
       weight: 1000,
@@ -151,7 +151,7 @@ async function setupFullCheckoutContext(
           productId,
           variantId,
           quantity: 2,
-          price: overrides?.price ?? 24.99,
+          price: overrides?.price ?? 2499,
         },
       ],
       createdAt: Date.now(),
@@ -294,7 +294,7 @@ describe("stripeActions.createCheckoutSession", () => {
       price_data: {
         currency: "gbp",
         product_data: { name: "Premium Kibble — 1kg Bag" },
-        unit_amount: Math.round(24.99 * 100),
+        unit_amount: 2499,
       },
       quantity: 2,
     });