feat(authorization): add authorization service

[ponchanon]

Summary

• Add application-facing IAuthorizationService
• Add AuthorizationService implementation
• Delegate authorization decisions to IAuthorizationEvaluator
• Add unit tests for allow, deny, validation, and delegation behavior

Validation

• dotnet build --no-incremental ✅
• dotnet test ✅ 24/24
• git diff --check ✅

Existing NU1903 warnings for Microsoft.OpenApi 2.0.0 remain unchanged.