[Testing] Improve handler test coverage (60 handlers, 4 tests)

[iammukeshm]

Problem

The codebase has 60 handler classes but only 4 handler test files.

Impact

• Low confidence in refactoring
• Potential regressions go undetected
• Missing documentation via tests

Handlers needing tests (priority)

1. Identity Module - Auth critical

   • GenerateTokenCommandHandler
   • RefreshTokenCommandHandler
   • RegisterUserCommandHandler
   • All session handlers

2. Multitenancy Module - Tenant operations

3. Auditing Module - Data integrity

Acceptance Criteria

• At least 50% handler test coverage
• All auth-related handlers tested

[iammukeshm]

Progress Update 🧪

Added handler tests for 5 auth-critical handlers in PR #1204:

Handler	Tests	Coverage
GenerateTokenCommandHandler	✅	Login flow
RefreshTokenCommandHandler	✅	Token refresh
RegisterUserCommandHandler	✅	User registration
RevokeSessionCommandHandler	✅	Session mgmt
ChangePasswordCommandHandler	✅	Password change

Status: 5/60+ handlers tested (auth-critical first)

Next batch can cover: Groups, Roles, and remaining Session handlers.