Fix failure to handle slnx files

Fixes #47038

Author: sharwell

src/BuiltInTools/dotnet-format/Workspaces/MSBuildWorkspaceFinder.cs

@@ -55,7 +55,9 @@ public static (bool isSolution, string workspacePath) FindWorkspace(string searc
         private static (bool isSolution, string workspacePath) FindFile(string workspacePath)
         {
             var workspaceExtension = Path.GetExtension(workspacePath);
-            var isSolution = workspaceExtension.Equals(".sln", StringComparison.OrdinalIgnoreCase) || workspaceExtension.Equals(".slnf", StringComparison.OrdinalIgnoreCase);
+            var isSolution = workspaceExtension.Equals(".sln", StringComparison.OrdinalIgnoreCase)
+                || workspaceExtension.Equals(".slnf", StringComparison.OrdinalIgnoreCase)
+                || workspaceExtension.Equals(".slnx", StringComparison.OrdinalIgnoreCase);
             var isProject = !isSolution
                 && workspaceExtension.EndsWith("proj", StringComparison.OrdinalIgnoreCase)
                 && !workspaceExtension.Equals(DnxProjectExtension, StringComparison.OrdinalIgnoreCase);

test/TestAssets/dotnet-format/for_workspace_finder/multiple_solutions/solution_c.slnx

@@ -0,0 +1 @@
+<Solution />

test/TestAssets/dotnet-format/for_workspace_finder/single_slnx/single_slnx.slnx

@@ -0,0 +1 @@
+<Solution />

test/dotnet-format.Tests/tests/MSBuild/MSBuildWorkspaceFinderTests.cs

@@ -77,6 +77,30 @@ public void FindsSolutionByFilePath()
             Assert.True(isSolution);
         }
 
+        [Fact]
+        public void FindsSlnxByFolder()
+        {
+            const string Path = "for_workspace_finder/single_slnx/";
+
+            var (isSolution, workspacePath) = MSBuildWorkspaceFinder.FindWorkspace(ProjectsPath, Path);
+
+            var solutionFileName = System.IO.Path.GetFileName(workspacePath);
+            Assert.Equal("single_slnx.slnx", solutionFileName);
+            Assert.True(isSolution);
+        }
+
+        [Fact]
+        public void FindsSlnxByFilePath()
+        {
+            const string Path = "for_workspace_finder/multiple_solutions/solution_c.slnx";
+
+            var (isSolution, workspacePath) = MSBuildWorkspaceFinder.FindWorkspace(ProjectsPath, Path);
+
+            var solutionFileName = System.IO.Path.GetFileName(workspacePath);
+            Assert.Equal("solution_c.slnx", solutionFileName);
+            Assert.True(isSolution);
+        }
+
         [Fact]
         public void FindsProjectByFolder()
         {