Add a Vitest unit test suite (cart totals, invoice tax grouping, bundle contents)

No test infrastructure existed in this repo yet. Covers the pure logic
most likely to silently produce wrong numbers on a live order: discount/
shipping math, per-rate invoice grouping, and bundle-contents string
building. Extracted describeBundleContents() out of the checkout route
into its own module so it's importable from a test (route.ts files only
allow HTTP-method exports).

package.json

@@ -6,7 +6,8 @@
     "dev": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test:unit": "vitest run"
   },
   "dependencies": {
     "@payloadcms/live-preview-react": "^3.85.2",
@@ -26,6 +27,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.2.9",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^4.1.10"
   }
 }