test: raise unit test coverage to 60 percent

[kklldog]

Summary

• add controller, OIDC, and common infrastructure unit coverage
• add Coverlet exclusions for pure models, DTOs, and contracts
• raise merged line coverage to 60.00% (5606/9343)

Verification

• dotnet vstest test\AgileConfig.Server.CommonTests\bin\Debug\net10.0\AgileConfig.Server.CommonTests.dll (22 passed)
• API: 44 passed; Application: 42 passed; Service SQLite: 40 passed

MongoDB Testcontainers tests require a running Docker daemon and were not run in this environment.