sln-add: Don't add solution folders outside the scope of solution directory (#48611)

Author: edvilme

src/Cli/dotnet/commands/dotnet-sln/add/Program.cs

@@ -100,8 +100,8 @@ private async Task AddProjectsToSolutionAsync(string solutionFileFullPath, IEnum
             foreach (var projectPath in projectPaths)
             {
                 string relativePath = Path.GetRelativePath(Path.GetDirectoryName(solutionFileFullPath), projectPath);
-                // Add fallback solution folder if relative path does not contain `..`.
-                string relativeSolutionFolder =  relativePath.Split(Path.DirectorySeparatorChar).Any(p => p == "..")
+                // Add auto-generated solution folder if relative path exists in the solution directory
+                string relativeSolutionFolder = (relativePath.Split(Path.DirectorySeparatorChar).Any(p => p == "..") || Path.IsPathRooted(relativePath))
                     ? string.Empty : Path.GetDirectoryName(relativePath);
 
                 if (!_inRoot && solutionFolder is null && !string.IsNullOrEmpty(relativeSolutionFolder))