Add deleteBackwardByDecomposingPreviousCharacter mapping for tests (#132919)

In  `MacOSTestTextInputKeyHandler`, there are some mappings from shortcuts to generate macOS selectors. However, `ctrl + backspace` isn't mapped to `deleteBackwardByDecomposingPreviousCharacter:`.

This PR adds the mapping for `deleteBackwardByDecomposingPreviousCharacter:`.

Fixes: https://github.com/flutter/flutter/issues/132917

packages/flutter_test/lib/src/test_text_input_key_handler.dart

@@ -50,6 +50,8 @@ class MacOSTestTextInputKeyHandler extends TestTextInputKeyHandler {
           alt: true, shift: pressShift): <String>['deleteWordBackward:'],
       SingleActivator(LogicalKeyboardKey.backspace,
           meta: true, shift: pressShift): <String>['deleteToBeginningOfLine:'],
+      SingleActivator(LogicalKeyboardKey.backspace, control: true, shift: pressShift):
+          <String>['deleteBackwardByDecomposingPreviousCharacter:'],
       SingleActivator(LogicalKeyboardKey.delete, shift: pressShift): <String>[
         'deleteForward:'
       ],

packages/flutter_test/test/test_text_input_test.dart

@@ -76,4 +76,26 @@ void main() {
       expect(selectorNames, isNull);
     }
   }, variant: TargetPlatformVariant.all());
+
+  testWidgets('selector is called for ctrl + backspace on macOS', (WidgetTester tester) async {
+    List<dynamic>? selectorNames;
+    await SystemChannels.textInput.invokeMethod('TextInput.setClient', <dynamic>[1, <String, dynamic>{}]);
+    await SystemChannels.textInput.invokeMethod('TextInput.show');
+    SystemChannels.textInput.setMethodCallHandler((MethodCall call) async {
+      if (call.method == 'TextInputClient.performSelectors') {
+        selectorNames = (call.arguments as List<dynamic>)[1] as List<dynamic>;
+      }
+    });
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.control);
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.backspace);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.backspace);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.control);
+    await SystemChannels.textInput.invokeMethod('TextInput.clearClient');
+
+    if (defaultTargetPlatform == TargetPlatform.macOS) {
+      expect(selectorNames, <dynamic>['deleteBackwardByDecomposingPreviousCharacter:']);
+    } else {
+      expect(selectorNames, isNull);
+    }
+  }, variant: TargetPlatformVariant.all());
 }