Migrate tests to Microsoft.Testing.Platform

[Evangelink]

Why

The test suite still depended on VSTest-specific runner configuration and command-line arguments. Moving every test project together to Microsoft.Testing.Platform enables the native .NET 10 test experience, direct executable test runs, and avoids the unsupported mixed-platform state.

What changed

• Enable the native Microsoft.Testing.Platform runner through global.json and shared MSTest runner properties.
• Build both test projects as executables and replace the VSTest coverage collector with the MTP code coverage extension.
• Update GitHub Actions and PowerShell scripts to use MTP coverage, GitHub reporting, and architecture arguments.
• Remove VSTest-only logger, collection, runsettings, and parallel switches.

Validation

• Release build succeeds with 0 warnings and 0 errors.
• Full MTP suite passes: 2,378 tests, 0 failures, 0 skipped.
• Direct Sep.XyzTest.exe execution passes 20 tests.
• Cobertura coverage artifacts are generated successfully.

Note

global.json remains pinned to stable .NET SDK 10.0.400. Local verification used the installed 10.0.400 preview SDK explicitly because the stable SDK was not available on the validation machine.