[TEST] Establish unit test project and 100% coverage on security-critical code

[cct08311github]

Create xUnit test project targeting critical security code:

• PasswordHashHelper (hash, verify, MD5 migration)
• TokenService (issue, refresh, revoke, reuse detection)
• RefreshTokenEntity (computed properties)

Goal: 100% test coverage on security-critical paths.
Risk: Low | Effort: 2 days