User/adstep/uwp net9 support

[adstep]

This PR updates (#3848) with the latest change from main.

#3848 includes the initial work to support UWP apps using .NET 9 (see here).
This tackles the extensions assembly:

• Multi-target net8.0-10.0.26100.0 (we can bump to .NET 9 once it goes GA)
• Define WINDOWS_UWP on this target
• Pack this .dll into net8.0-windows10.0.26100.0/uwp
• Update .targets to reference it if UseUwpTools is set (ie. this is a UWP app using .NET 9)

[Youssef1313]

Is there another PR that touches VSTest? If so, we will need to later update the sample with a version available on NuGet instead of a locally built version

[adstep]

Yea I had some changes in vstest to remove subscribing to the process, which doesn't work in UWP. @Evangelink is working on a PR for that. I'll circle back and update these to the public version when that gets published. And also minimize what's needed.

Sergio originally had which I think should be sufficient (plus removing the dev dependency again):

    <PackageReference Include="Microsoft.Windows.SDK.CPP" Version="10.0.26100.2454" />

	<!-- Do not use MSTest metapackage as we don't want to use Microsoft.NET.Test.Sdk -->
	<PackageReference Include="MSTest.TestFramework" Version="3.7.0-dev" />
	<PackageReference Include="MSTest.TestAdapter" Version="3.7.0-dev" />
	<PackageReference Include="MSTest.Analyzers" Version="3.7.0-dev" />
	<!-- Instead we want to rely on Microsoft.TestPlatform.TestHost to support msix -->
	<PackageReference Include="Microsoft.TestPlatform.TestHost" Version="17.12.0" />

[Evangelink]

@adstep I don't recall if you did some changes to VSTest or not.

[adstep]

@Evangelink I had to remove UnitTestClient from subscribing to Parent process' ExitCallback. ProcessHelper throws in the UWP setting because it executes in a sandbox that doesn't have permissions to access process info.

The old VSTest framework that works with legacy UWP just had ProcessHelper as a complete no-op for all the methods.

If you remove the ExitCallabck then it leaves the UnitTestClient open to hanging potentially - but it got the tests running for me. I wasn't sure if the old VSTest framework did something diifferent for legacy UWP.

[Youssef1313]

nit: Indentation is different

[adstep]

I'm struggling to fix this in my editor. If you can send a workspace commit I'll update it.

[Youssef1313]

How are we differentiating net9.0 UWP vs net9.0 WinUI? They both use the same TFM right?

[adstep]

I believe the plan was to use UseWinUI and UseUwpTools properties.

[Evangelink]

Let me done some of these fixes in a separate PR.

[Evangelink]

Closing this one in favor of the initial from @Sergio0694