Fix icon database load on .NET 10

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• stop reusing the shared JSON serializer options when constructing the updater source-generated context
• keep the shared serializer options resolver generic so the icon screenshot database can still deserialize on .NET 10
• add a regression meta-test to prevent future JsonSerializerContext construction from rebinding the shared options instance

Validation

• dotnet test 'src/UniGetUI.Core.Tools.Tests/UniGetUI.Core.Tools.Tests.csproj' --no-restore --verbosity minimal
• dotnet build 'src/UniGetUI/UniGetUI.csproj' --no-restore -v q