Support trailing commas and comments in .slnf files

[Copilot]

• Replace net8.0 with $(CurrentTargetFramework) in App.csproj and Lib.csproj
• Update .github/copilot-instructions.md to document using $(CurrentTargetFramework) for test projects
• 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 dotnet/msbuild#6346

sdk/src/Cli/dotnet/SlnFileFactory.cs

Line 92 in f578225

JsonElement root = JsonDocument.Parse(File.ReadAllText(filteredSolutionPath)).RootElement;

@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 Revise CreateFromFilteredSolutionFile implementation for trailing comma (and allowing comments) #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.

[Copilot]

Pull request overview

This PR adds support for trailing commas and JSON comments in solution filter (.slnf) files, aligning the SDK's behavior with MSBuild's support added in dotnet/msbuild#6346. The change modifies the JSON parsing in SlnFileFactory.CreateFromFilteredSolutionFile to allow these relaxed JSON features, improving developer experience when editing .slnf files.

Key Changes

• Modified SlnFileFactory.cs to pass JsonDocumentOptions with AllowTrailingCommas = true and CommentHandling = JsonCommentHandling.Skip when parsing .slnf files
• Added comprehensive test coverage with two new test cases validating both trailing commas and comments work correctly
• Created TestAppWithTrailingCommaSlnf test asset with two .slnf files demonstrating both features

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Cli/dotnet/SlnFileFactory.cs	Configured JsonDocumentOptions to allow trailing commas and skip comments when parsing .slnf files
test/dotnet.Tests/CommandTests/Solution/List/GivenDotnetSlnList.cs	Added two parameterized tests validating both "sln" and "solution" commands work with trailing commas and comments in .slnf files
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/AppWithTrailingComma.slnf	Test asset demonstrating .slnf with trailing comma and comments
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/AppWithComments.slnf	Test asset demonstrating .slnf with multi-line and single-line comments
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/App.sln	Base solution file referenced by the test .slnf files
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/App/App.csproj	App project referenced in the test solution
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/App/Program.cs	Simple console app for test asset
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/Lib/Lib.csproj	Library project referenced in the test solution
test/TestAssets/TestProjects/TestAppWithTrailingCommaSlnf/Lib/LibClass.cs	Simple library class for test asset