Add functional tests for Next.js

[chalosalvador]

Add functional tests for Next.js adapter

This PR introduces functional tests for the Next.js adapter and makes minor improvements to the starter template.

Changes

• Added tests for packages/@apphosting/adapter-nextjs:
  • e2e/app.spec.ts with comprehensive tests for various Next.js features
  • e2e/run-local.ts to set up and run the test environment
  • package.json with new test scripts for unit and functional testing
• Fixed TypeScript errors in the starter template:
  • Added void to onClick handler in RegenerateButton.tsx
  • Removed async from the route handler in route.ts

Testing

To run the new functional tests:

npm run test:functional

This will build the project, set up a test environment, and run the tests against a local Next.js server.