Fix NullReferenceException in dotnet sln remove for .slnx files with invalid path

[Copilot]

dotnet sln remove <invalid> on a non-empty .slnx solution throws NullReferenceException instead of printing the expected "project not found" message.

Root cause

The fallback name-matching path (used when no extension is present) called:

// p.DisplayName is null for .slnx projects — no explicit display name in the format
Path.GetFileNameWithoutExtension(p.DisplayName).Equals(projectPath)
//                                ^null          ^ NullReferenceException

.slnx projects don't carry an explicit display name, so p.DisplayName is null. Path.GetFileNameWithoutExtension(null) returns null, and calling .Equals() on it throws.

Fix

• SolutionRemoveCommand.cs — invert the receiver so null is the argument, not the receiver:

  projectPath.Equals(Path.GetFileNameWithoutExtension(p.DisplayName))

  String.Equals(null) returns false, so unresolved display names fall through cleanly to the "not found" output.

• GivenDotnetSlnRemove.cs — add WhenPassedAReferenceWithoutExtensionNotInSlnItPrintsStatus covering all four sln/solution × .sln/.slnx combinations.

[marcpopMSFT]

I didn't test in codespace as this looks pretty straightforward as does the test. I had copilot double check and a null projectpath would have errored earlier in the flow so this change should be safe.

[Copilot]

Pull request overview

Fixes a NullReferenceException in dotnet sln remove when operating on non-empty .slnx solutions and the user passes a project reference without an extension that doesn’t exist in the solution.

Changes:

• Avoid NullReferenceException in the “match by name without extension” fallback path in SolutionRemoveCommand.
• Add a new test covering the “no extension + not in solution” case across sln/solution and .sln/.slnx.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Cli/dotnet/Commands/Solution/Remove/SolutionRemoveCommand.cs	Updates the name-matching fallback used when no extension is present.
test/dotnet.Tests/CommandTests/Solution/Remove/GivenDotnetSlnRemove.cs	Adds coverage ensuring “project not found” output for missing no-extension references across .sln and .slnx.