chore(test): migrate to Microsoft.Testing.Platform

[j-d-ha]

Summary

Migrate the test projects from xUnit's legacy VSTest-based runner to the new Microsoft.Testing.Platform.

Background

Microsoft.Testing.Platform is the modern replacement for VSTest, offering:

• Better performance and startup times
• Native integration with modern .NET tooling
• Improved extensibility model
• Better support for cloud and container scenarios

Tasks

• Update test project files to use Microsoft.Testing.Platform
• Replace xUnit VSTest runner with xUnit Microsoft.Testing.Platform adapter
• Update any test tooling dependencies (AutoFixture.Xunit3, etc.) if needed
• Verify all tests run correctly with the new platform
• Update CI/CD pipelines if necessary
• Update documentation if test running commands change

Acceptance Criteria

• All existing tests pass using Microsoft.Testing.Platform
• `dotnet test` continues to work as expected
• CI/CD workflows continue to run tests successfully
• No test functionality is lost in the migration

References

• Microsoft.Testing.Platform documentation
• xUnit migration guide