feat(authorization): add authorization pipeline

[ponchanon]

Summary

• Add the authorization pipeline abstraction
• Add authorization request model
• Integrate policy evaluation with authorization service
• Add comprehensive authorization pipeline unit tests

Validation

• dotnet build --no-incremental ✅
• dotnet test — 29 passed, 0 failed ✅
• git diff --check ✅

Existing Microsoft.OpenApi NU1903 warnings remain unchanged.