[Android] Add custom system-wide text selection toolbar buttons for SelectableRegion (#141103)

## Description

This PR adds custom system-wide text selection toolbar buttons on Android for `SelectableRegion` and `SelectionArea`.

https://github.com/flutter/flutter/pull/139738 adds those buttons for `EditableText` (which is used by `TextField` and `SelectableText` but not by `SelectionArea`).

## Related Issue

Step 5 for https://github.com/flutter/flutter/issues/139361

## Tests

Adds 2 tests.

packages/flutter/lib/src/widgets/selectable_region.dart

@@ -331,6 +331,12 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
   @visibleForTesting
   SelectionOverlay? get selectionOverlay => _selectionOverlay;
 
+  /// The text processing service used to retrieve the native text processing actions.
+  final ProcessTextService _processTextService = DefaultProcessTextService();
+
+  /// The list of native text processing actions provided by the engine.
+  final List<ProcessTextAction> _processTextActions = <ProcessTextAction>[];
+
   @override
   void initState() {
     super.initState();
@@ -359,6 +365,14 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
         instance.onSecondaryTapDown = _handleRightClickDown;
       },
     );
+    _initProcessTextActions();
+  }
+
+  /// Query the engine to initialize the list of text processing actions to show
+  /// in the text selection toolbar.
+  Future<void> _initProcessTextActions() async {
+    _processTextActions.clear();
+    _processTextActions.addAll(await _processTextService.queryTextActions());
   }
 
   @override
@@ -1203,7 +1217,29 @@ class SelectableRegionState extends State<SelectableRegion> with TextSelectionDe
             hideToolbar();
         }
       },
-    );
+    )..addAll(_textProcessingActionButtonItems);
+  }
+
+  List<ContextMenuButtonItem> get _textProcessingActionButtonItems {
+    final List<ContextMenuButtonItem> buttonItems = <ContextMenuButtonItem>[];
+    final SelectedContent? data = _selectable?.getSelectedContent();
+    if (data == null) {
+      return buttonItems;
+    }
+
+    for (final ProcessTextAction action in _processTextActions) {
+      buttonItems.add(ContextMenuButtonItem(
+        label: action.label,
+        onPressed: () async {
+          final String selectedText = data.plainText;
+          if (selectedText.isNotEmpty) {
+            await _processTextService.processTextAction(action.id, selectedText, true);
+            hideToolbar();
+          }
+        },
+      ));
+    }
+    return buttonItems;
   }
 
   /// The line height at the start of the current selection.

packages/flutter/test/material/selection_area_test.dart

@@ -7,8 +7,10 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../widgets/process_text_utils.dart';
 
 Offset textOffsetToPosition(RenderParagraph paragraph, int offset) {
   const Rect caret = Rect.fromLTWH(0.0, 0.0, 2.0, 20.0);
@@ -200,6 +202,54 @@ void main() {
     skip: kIsWeb, // [intended]
   );
 
+  testWidgets('Text processing actions are added to the toolbar', (WidgetTester tester) async {
+    final MockProcessTextHandler mockProcessTextHandler = MockProcessTextHandler();
+    TestWidgetsFlutterBinding.ensureInitialized().defaultBinaryMessenger
+        .setMockMethodCallHandler(SystemChannels.processText, mockProcessTextHandler.handleMethodCall);
+    addTearDown(() => tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.processText, null));
+
+    final FocusNode focusNode = FocusNode();
+    addTearDown(focusNode.dispose);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: SelectionArea(
+          focusNode: focusNode,
+          child: const Text('How are you?'),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsNothing);
+
+    final RenderParagraph paragraph = tester.renderObject<RenderParagraph>(
+      find.descendant(
+        of: find.text('How are you?'),
+        matching: find.byType(RichText),
+      ),
+    );
+    final TestGesture gesture = await tester.startGesture(textOffsetToPosition(paragraph, 6)); // at the 'r'
+    addTearDown(gesture.removePointer);
+    await tester.pump(const Duration(milliseconds: 500));
+    // `are` is selected.
+    expect(paragraph.selections[0], const TextSelection(baseOffset: 4, extentOffset: 7));
+
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    // The toolbar is visible.
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
+
+    // The text processing actions are visible on Android only.
+    final bool areTextActionsSupported = defaultTargetPlatform == TargetPlatform.android;
+    expect(find.text(fakeAction1Label), areTextActionsSupported ? findsOneWidget : findsNothing);
+    expect(find.text(fakeAction2Label), areTextActionsSupported ? findsOneWidget : findsNothing);
+  },
+    variant: TargetPlatformVariant.all(),
+    skip: kIsWeb, // [intended]
+  );
+
   testWidgets('onSelectionChange is called when the selection changes', (WidgetTester tester) async {
     SelectedContent? content;
 

packages/flutter/test/widgets/selectable_region_test.dart

@@ -11,6 +11,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 import 'clipboard_utils.dart';
 import 'keyboard_utils.dart';
+import 'process_text_utils.dart';
 import 'semantics_tester.dart';
 
 Offset textOffsetToPosition(RenderParagraph paragraph, int offset) {
@@ -3368,6 +3369,60 @@ void main() {
     skip: kIsWeb, // [intended]
   );
 
+  testWidgets('Text processing actions are added to the toolbar', (WidgetTester tester) async {
+    final MockProcessTextHandler mockProcessTextHandler = MockProcessTextHandler();
+    TestWidgetsFlutterBinding.ensureInitialized().defaultBinaryMessenger
+        .setMockMethodCallHandler(SystemChannels.processText, mockProcessTextHandler.handleMethodCall);
+    addTearDown(() => tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.processText, null));
+
+    Set<String?> buttonLabels = <String?>{};
+    final FocusNode focusNode = FocusNode();
+    addTearDown(focusNode.dispose);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: SelectableRegion(
+          focusNode: focusNode,
+          selectionControls: materialTextSelectionHandleControls,
+          contextMenuBuilder: (
+            BuildContext context,
+            SelectableRegionState selectableRegionState,
+          ) {
+            buttonLabels = selectableRegionState.contextMenuButtonItems
+              .map((ContextMenuButtonItem buttonItem) => buttonItem.label)
+              .toSet();
+            return const SizedBox.shrink();
+          },
+          child: const Text('How are you?'),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    final RenderParagraph paragraph = tester.renderObject<RenderParagraph>(
+      find.descendant(
+        of: find.text('How are you?'),
+        matching: find.byType(RichText),
+      ),
+    );
+    final TestGesture gesture = await tester.startGesture(textOffsetToPosition(paragraph, 6)); // at the 'r'
+    addTearDown(gesture.removePointer);
+    await tester.pump(const Duration(milliseconds: 500));
+    // `are` is selected.
+    expect(paragraph.selections[0], const TextSelection(baseOffset: 4, extentOffset: 7));
+
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    // The text processing actions are available on Android only.
+    final bool areTextActionsSupported = defaultTargetPlatform == TargetPlatform.android;
+    expect(buttonLabels.contains(fakeAction1Label), areTextActionsSupported);
+    expect(buttonLabels.contains(fakeAction2Label), areTextActionsSupported);
+  },
+    variant: TargetPlatformVariant.all(),
+    skip: kIsWeb, // [intended]
+  );
+
   testWidgets('onSelectionChange is called when the selection changes through gestures', (WidgetTester tester) async {
     SelectedContent? content;
     final FocusNode focusNode = FocusNode();