Enhanced test to verify .config manifest update and root directory checks

Co-authored-by: dsplaisted <145043+dsplaisted@users.noreply.github.com>

Author: Copilot

test/dotnet.Tests/CommandTests/Tool/Install/ToolInstallLocalCommandTests.cs

@@ -333,6 +333,15 @@ public void WhenRunWithExistingManifestInConfigDirectoryItShouldAddToExistingMan
             var manifestPackages = _toolManifestFinder.Find();
             manifestPackages.Should().HaveCount(1);
             manifestPackages.First().PackageId.Should().Be(_packageIdA);
+
+            // Verify that the manifest under the .config folder has been updated
+            _fileSystem.File.Exists(configManifestPath).Should().BeTrue("the .config manifest file should exist");
+            var configManifestContent = _fileSystem.File.ReadAllText(configManifestPath);
+            configManifestContent.Should().Contain(_packageIdA.ToString(), "the .config manifest should contain the installed tool");
+            configManifestContent.Should().NotBe(_jsonContent, "the .config manifest should have been updated with the new tool");
+
+            // Verify that no manifest exists in the root folder after the install command is run
+            _fileSystem.File.Exists(_manifestFilePath).Should().BeFalse("no manifest should exist in the root folder");
         }
 
         private ToolInstallLocalCommand GetDefaultTestToolInstallLocalCommand()