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fix(test): switch E2E tests from identity proxy header to session cookie auth

Replace identity proxy header (X-Frontier-Email) authentication with
proper mail OTP + session cookie auth flow using test_users config.
This fixes all E2E test failures after migrating to ConnectRPC clients,
since the ConnectRPC server doesn't support identity proxy headers.

Key changes:
- Add AuthenticateUser() and ContextWithAuth() helpers in testbench
- Update all Bootstrap functions to accept session cookie instead of email
- Configure test_users + mail OTP in all test suite configs
- Add Session.Validity to prevent immediate session expiry
- Fix authentication_test.go to use Set-Cookie and Bearer token headers
- Fix non-raystack.org email domains to work with test_users config
- Delete passthrough_header_test.go (no longer applicable)
- Remove IdentityHeader constant (no longer needed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>