Fix Shortcut label for CharacterActivator does not include modifiers (#152233)

## Description

This PR fixes the shortcut label for CharacterActivator with modifiers keys.

**Before**:

![image](https://github.com/user-attachments/assets/1cb8defe-2600-45ef-878d-fbdde5aaf2ad)

**After**:

![image](https://github.com/user-attachments/assets/cd3b1c79-2f23-40fd-b0b9-934fa182fff3)

## Related Issue

Fixes flutter/flutter#145040.

## Tests

Adds 1 test.

Author: bleroux

packages/flutter/lib/src/material/menu_anchor.dart

@@ -2156,7 +2156,7 @@ class _LocalizedShortcutLabeler {
       final LogicalKeyboardKey trigger = serialized.trigger!;
       final List<String> modifiers = <String>[
         if (_usesSymbolicModifiers) ...<String>[
-          // MacOS/iOS platform convention uses this ordering, with ⌘ always last.
+          // macOS/iOS platform convention uses this ordering, with ⌘ always last.
           if (serialized.control!) _getModifierLabel(LogicalKeyboardKey.control, localizations),
           if (serialized.alt!)     _getModifierLabel(LogicalKeyboardKey.alt, localizations),
           if (serialized.shift!)   _getModifierLabel(LogicalKeyboardKey.shift, localizations),
@@ -2190,7 +2190,24 @@ class _LocalizedShortcutLabeler {
         if (shortcutTrigger != null && shortcutTrigger.isNotEmpty) shortcutTrigger,
       ].join(keySeparator);
     } else if (serialized.character != null) {
-      return serialized.character!;
+      final List<String> modifiers = <String>[
+        // Character based shortcuts cannot check shifted keys.
+        if (_usesSymbolicModifiers) ...<String>[
+          // macOS/iOS platform convention uses this ordering, with ⌘ always last.
+          if (serialized.control!) _getModifierLabel(LogicalKeyboardKey.control, localizations),
+          if (serialized.alt!)     _getModifierLabel(LogicalKeyboardKey.alt, localizations),
+          if (serialized.meta!)    _getModifierLabel(LogicalKeyboardKey.meta, localizations),
+        ] else ...<String>[
+          // This order matches the LogicalKeySet version.
+          if (serialized.alt!)     _getModifierLabel(LogicalKeyboardKey.alt, localizations),
+          if (serialized.control!) _getModifierLabel(LogicalKeyboardKey.control, localizations),
+          if (serialized.meta!)    _getModifierLabel(LogicalKeyboardKey.meta, localizations),
+        ],
+      ];
+      return <String>[
+        ...modifiers,
+        serialized.character!,
+      ].join(keySeparator);
     }
     throw UnimplementedError('Shortcut labels for ShortcutActivators that do not implement '
         'MenuSerializableShortcut (e.g. ShortcutActivators other than SingleActivator or '

packages/flutter/test/material/menu_anchor_test.dart

@@ -2201,6 +2201,58 @@ void main() {
       skip: kIsWeb && !isCanvasKit, // https://github.com/flutter/flutter/issues/145527
     );
 
+    // Regression test for https://github.com/flutter/flutter/issues/145040.
+    testWidgets('CharacterActivator shortcut mnemonics include modifiers', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: MenuBar(
+              controller: controller,
+              children: createTestMenus(
+                shortcuts: <TestMenu, MenuSerializableShortcut>{
+                  TestMenu.subSubMenu110: const CharacterActivator('A', control: true),
+                  TestMenu.subSubMenu111: const CharacterActivator('B', alt: true),
+                  TestMenu.subSubMenu112: const CharacterActivator('C', meta: true),
+                },
+              ),
+            ),
+          ),
+        ),
+      );
+
+      // Open a menu initially.
+      await tester.tap(find.text(TestMenu.mainMenu1.label));
+      await tester.pump();
+
+      await tester.tap(find.text(TestMenu.subMenu11.label));
+      await tester.pump();
+
+      final Text mnemonic0 = tester.widget(findMnemonic(TestMenu.subSubMenu110.label));
+      final Text mnemonic1 = tester.widget(findMnemonic(TestMenu.subSubMenu111.label));
+      final Text mnemonic2 = tester.widget(findMnemonic(TestMenu.subSubMenu112.label));
+
+      switch (defaultTargetPlatform) {
+        case TargetPlatform.android:
+        case TargetPlatform.fuchsia:
+        case TargetPlatform.linux:
+          expect(mnemonic0.data, equals('Ctrl+A'));
+          expect(mnemonic1.data, equals('Alt+B'));
+          expect(mnemonic2.data, equals('Meta+C'));
+        case TargetPlatform.windows:
+          expect(mnemonic0.data, equals('Ctrl+A'));
+          expect(mnemonic1.data, equals('Alt+B'));
+          expect(mnemonic2.data, equals('Win+C'));
+        case TargetPlatform.iOS:
+        case TargetPlatform.macOS:
+          expect(mnemonic0.data, equals('⌃ A'));
+          expect(mnemonic1.data, equals('⌥ B'));
+          expect(mnemonic2.data, equals('⌘ C'));
+      }
+    },
+      variant: TargetPlatformVariant.all(),
+      skip: kIsWeb && !isCanvasKit, // https://github.com/flutter/flutter/issues/145527
+    );
+
     testWidgets('leadingIcon is used when set', (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(