test(#411): add missing test coverage — getAccessToken, isAdmin, null cases, getClientIp
- Add getAccessToken tests (5): null session, valid token, expired token, buffer window, undefined token
- Add isAdmin tests (4): null session, true, false, undefined
- Add getUserById/getUserByEmail null-return tests (2)
- Add getClientIp tests via handleAuth (4): single IP, comma-separated, array, fallback
- Fix pre-existing controller spec failure by adding better-auth vi.mock calls
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>