Support trailing commas and comments in .slnf files

[Copilot]

• [x] Replace net8.0 with $(CurrentTargetFramework) in App.csproj and Lib.csproj
• [x] Update .github/copilot-instructions.md to document using $(CurrentTargetFramework) for test projects
• [x] Verify all tests still pass (4/4 tests passed)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Revise CreateFromFilteredSolutionFile implementation for trailing comma (and allowing comments)</issue_title> <issue_description>The current implementation of this method doesn't seem to allow trailing commas (and comments).

MSBuild supported trailing commas (and comments) of slnf files in https://github.com/dotnet/msbuild/pull/6346

https://github.com/dotnet/sdk/blob/f578225fe1fdd93db9efac273562b3709db9e8e9/src/Cli/dotnet/SlnFileFactory.cs#L92

@baronfel @rainersigwald Another discrepancy due to not having a unified implementation/helper for how to parse solutions and solution filters. Not specific to test command this time :) </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes dotnet/sdk#51929

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[marcpopMSFT]

@copilot please replace the two new net8.0 target frameworks you added with $(CurrentTargetFramework). Please update the copilot instructions to let future copilots know to always use $(CurrentTargetFramework) by default when creating new test projects.

[marcpopMSFT]

Tested in codespaces with a slnf that had trailing commas and comments.

[Copilot]

@copilot please replace the two new net8.0 target frameworks you added with $(CurrentTargetFramework). Please update the copilot instructions to let future copilots know to always use `$(CurrentTarg...

Done in 5fac8cb. Replaced both instances of net8.0 with $(CurrentTargetFramework) and added the guidance to the copilot instructions file.