Handle TypeLoadException in config property scans

[duskfold]

Summary

• handle TypeLoadException while checking foreign properties for ConfigurationPropertyAttribute
• expose the assembly-scan helper so the failure can be proven directly in tests
• add deterministic fixture-backed proof for the custom-attribute metadata failure shape

Why

This supersedes the closed #5124 thread with the corrected proof.

The failure is not a plain Assembly.GetTypes() repro on modern CoreCLR. The real downstream failure happens later in the same config-host scan, when ConfigProperty.Initialize() reflects over properties from foreign loaded assemblies and PropertyInfo.GetCustomAttribute(...) throws a plain TypeLoadException while resolving broken custom-attribute metadata.

The new test proves that exact shape with two fixture assemblies:

• ConsumerLib.dll is compiled against a provider assembly that contains System.Diagnostics.CodeAnalysis.MemberNotNullWhenAttribute
• at runtime the test loads a same-identity replacement provider assembly that no longer contains that type
• reflecting GetCustomAttribute(typeof(ConfigurationPropertyAttribute)) on the consumer property throws TypeLoadException
• ConfigProperty.ScanAssembliesForConfigPropertyHosts(...) now skips that foreign property and continues scanning later assemblies

That matches the downstream MemberNotNullWhenAttribute failure we were seeing from loaded test-platform assemblies, but reduces it to a deterministic upstream test.

Validation

• dotnet restore
• dotnet build --no-restore
• dotnet test --project Tests/UnitTests.NonParallelizable/UnitTests.NonParallelizable.csproj --no-build -- --filter-class UnitTests.NonParallelizable.ConfigurationTests.ConfigPropertyAssemblyScanTests
• dotnet test --project Tests/UnitTests.NonParallelizable/UnitTests.NonParallelizable.csproj --no-build -- --filter-namespace UnitTests.NonParallelizable.ConfigurationTests
• dotnet test --project Tests/UnitTestsParallelizable/UnitTests.Parallelizable.csproj --no-build -- --filter-namespace ConfigurationTests
• dotnet test --project Tests/UnitTests.NonParallelizable/UnitTests.NonParallelizable.csproj --no-build
• dotnet test --project Tests/UnitTestsParallelizable/UnitTests.Parallelizable.csproj --no-build