- Test successful updates (5, 10, 50, 100 devices)
- Test validation failures (0, -1, 101, 1000 devices)
- Test authentication requirements (no token, non-admin)
- Test non-existent user ID
- Test missing user ID in URL
- Test max_devices field in user list response
- Add 20+ test cases across 7 test functions
- Helper functions for admin user creation and login