Add validation error details to ProblemDetailsModel

[tiksn]

Summary

• Add errors and traceId to ProblemDetailsModel so validation responses deserialize cleanly.
• Update shared test fixtures to populate the expanded model shape.
• Add serializer coverage for RFC 7807 validation payloads, including nested field errors.

Testing

• dotnet test .\tests\Bridge.Tests\Bridge.Tests.fsproj
• .�pack.ps1