Update xunit templates to xunit.v3

[Youssef1313]

Fixes #45645

[Copilot]

Pull Request Overview

This PR updates the xUnit templates to use xUnit v3 by replacing the xunit package references with xunit.v3 across all project templates and adding Microsoft.Testing.Platform configuration guidance.

Key Changes

• Updated package references from xunit version 2.9.3 to xunit.v3 version 3.0.0
• Added commented configuration options and documentation for Microsoft.Testing.Platform integration
• Applied changes consistently across C#, F#, and Visual Basic project templates

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Company.TestProject1.csproj	Updated C# xUnit template with v3 package and Microsoft.Testing.Platform documentation
Company.TestProject1.fsproj	Updated F# xUnit template with v3 package and Microsoft.Testing.Platform documentation
Company.TestProject1.vbproj	Updated VB xUnit template with v3 package and Microsoft.Testing.Platform documentation

[bradwilson]

Is there any reason these templates shouldn't be exactly what we're shipping in xunit.v3.templates?

[bradwilson]

What I'm saying here is: I like my templates a lot better than yours. Shipping them was not just a holdover waiting for you to update yours.

[Youssef1313]

@bradwilson Is it better now? Any other concerns you have with the current shape? Maybe we could add xunit.runner.json similar to your templates. Anything other than that?

[bradwilson]

I would love for you to include a default xunit.runner.json like we do. That's a pretty big usability win for us because we include the schema in the JSON file which gives them intellisense in all the major IDEs.

[Youssef1313]

I agree!

[bradwilson]

We also ship templates for extension authors, but I'm guessing that's probably beyond scope for inclusion here (which means we'll still end up shipping our templates; that'll need to be true until net8 is out of support anyways, since these new templates are only going in the net10 SDK).

[bradwilson]

For the xunit.runner.json file, just this is enough:

{
    "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json"
}

Along with this in the project file:

  <ItemGroup>
    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
  </ItemGroup

It's applicable to all languages.