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

vitest.config.ts

@@ -1,6 +1,12 @@
+import path from "node:path";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "apps/storefront/src"),
+    },
+  },
   test: {
     environment: "edge-runtime",
     server: { deps: { inline: ["convex-test"] } },