Update test project to target net10.0 #704

Problem

The test project currently targets net8.0 and takes dependencies on Microsoft.NET.Test.Sdk and xunit.runner.visualstudio which are implicitly referenced by SDK 8.0.420, producing NETSDK1023 warnings during build. However, removing the explicit references causes the test host to crash because the implicit versions don't properly resolve the Argon transitive dependency required by Verify.Xunit.

.NET 8 reaches end of life in November 2026, so the repo will need to move forward regardless.

Suggestion

Update the test project's target framework from net8.0 to net10.0 (or newer). This would:

• Align the test infrastructure with a supported, long-term .NET version
• Likely resolve the implicit vs explicit package reference conflict cleanly
• Allow removal of the now-redundant explicit Microsoft.NET.Test.Sdk and xunit.runner.visualstudio references
• The CI pipeline's UseDotNet@2 task (version: 8.x) would also need to be updated accordingly