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/carts.test.ts

@@ -26,6 +26,8 @@ async function setupUserAndVariant(t: ReturnType<typeof convexTest>) {
       status: "active",
       categoryId,
       tags: [],
+      parentCategorySlug: "toys",
+      childCategorySlug: "toys",
     });
     variantId = await ctx.db.insert("productVariants", {
       productId,
@@ -35,6 +37,8 @@ async function setupUserAndVariant(t: ReturnType<typeof convexTest>) {
       stockQuantity: 50,
       attributes: { color: "Red" },
       isActive: true,
+      weight: 100,
+      weightUnit: "g",
     });
   });