Add status code helpers and make ProblemDetails status non-nullable

[tiksn]

Summary

• Add focused HTTP status helpers for 4xx and 5xx detection.
• Make ProblemDetailsModel.Status a non-nullable int.
• Add tests for status-code boundaries, ProblemDetailsModel helpers, and JSON serialization/deserialization.

Testing

• Added unit coverage for isStatusCodeClientError and isStatusCodeServerError at range boundaries.
• Added unit coverage for isClientProblem and isServerProblem using ProblemDetailsModel instances.
• Updated serializer tests for non-nullable Status and verified missing status deserializes to the default 0.
• dotnet test passed locally.