Fix duplicate solution folder creation when adding multiple projects from same directory

[Copilot]

When adding multiple projects from the same directory (e.g., dotnet sln add Dir/Proj1.csproj Dir/Proj2.csproj), GenerateIntermediateSolutionFoldersForProjectPath would call solution.AddFolder() for each project without checking if the folder already exists, causing duplicate folder entries.

Changes

• SolutionAddCommand.cs: Check for existing solution folder before creating new one

  • Use SingleOrDefault() to find existing folder by path
  • Reuse existing folder if found, create new one only if needed

• GivenDotnetSlnAdd.cs: Add test WhenMultipleProjectsFromSameDirectoryAreAddedSolutionFolderIsNotDuplicated

  • Validates both projects end up in same solution folder
  • Covers .sln and .slnx formats

// Before: Always created new folder, causing duplicates
return solution.AddFolder(GetSolutionFolderPathWithForwardSlashes(relativeSolutionFolderPath));

// After: Reuse existing folder if present
var solutionFolderPath = GetSolutionFolderPathWithForwardSlashes(relativeSolutionFolderPath);
var existingFolder = solution.SolutionFolders.SingleOrDefault(f => f.Path == solutionFolderPath);
return existingFolder ?? solution.AddFolder(solutionFolderPath);

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Regression] dotnet sln add with 2 projects on the same directory produces an invalid solution</issue_title>
<issue_description>### Describe the bug

Creating a solution using dotnet sln add with 2 projects presents on the same directory produces a non working solution. The error is the following as when opening the solution in VS2022:

It has been present since when I upgraded to (around) VS 2022 17.14.19. I can reproduce with the last VS 2022 maintenance version 17.14.21, which has dotnet SDK 9.0.308 and 8.0.416. I have a linux box that ships with older SDKs 9.0.107/8.0.117 where I don't reproduce.

To Reproduce

Uncompress TestDotnetSlnAddProject.zip and run bootstrap.ps1 which does:

dotnet new sln --name TestDotnetSlnAddProject --force
dotnet sln TestDotnetSlnAddProject.sln add TestDotnetSlnAddProject/TestDotnetSlnAddProject.csproj TestDotnetSlnAddProject/TestDotnetSlnAddProject2.csproj
dotnet restore TestDotnetSlnAddProject.sln

Exceptions (if any)

dotnet restore outputs:

Solution file error MSB5004: The solution file has two projects named "TestDotnetSlnAddProject".

Opening in Visual Studio:

"The solution folder cannot be added to the solution because a Solution Folder with the same unique identifier already exits"

Further technical details

details of dotnet --info

• Fixes [Regression] dotnet sln add with 2 projects on the same directory produces an invalid solution #51970

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