security: bump next to 15.2.6 in next-js-15 example

Addresses critical advisories for Next.js 15.x including patched releases
for GHSA-f82v-jwr5-mffw (middleware) and GHSA-9qr9-h5gf-34mp (React flight).

Made-with: Cursor

Author: thomasrockhu-codecov

examples/next-js-15/package.json

@@ -9,7 +9,7 @@
     "lint": "next lint"
   },
   "dependencies": {
-    "next": "15.1.0",
+    "next": "15.2.6",
     "react": "19.0.0",
     "react-dom": "19.0.0"
   },