RenderParagraph should dispose instances of SelectableFragments. (#133915)

Author: polina-c

packages/flutter/lib/src/rendering/paragraph.dart

@@ -437,9 +437,7 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
   @override
   void dispose() {
     _removeSelectionRegistrarSubscription();
-    // _lastSelectableFragments may hold references to this RenderParagraph.
-    // Release them manually to avoid retain cycles.
-    _lastSelectableFragments = null;
+    _disposeSelectableFragments();
     _textPainter.dispose();
     super.dispose();
   }

packages/flutter/test/material/elevated_button_test.dart

@@ -1684,7 +1684,7 @@ void main() {
     expect(RendererBinding.instance.mouseTracker.debugDeviceActiveCursor(1), SystemMouseCursors.basic);
   });
 
-  testWidgets('ElevatedButton in SelectionArea changes mouse cursor when hovered', (WidgetTester tester) async {
+  testWidgetsWithLeakTracking('ElevatedButton in SelectionArea changes mouse cursor when hovered', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/104595.
     await tester.pumpWidget(MaterialApp(
       home: SelectionArea(