Fix not-disposed _InputBorderGap. (#132694)

Fixes: https://github.com/flutter/flutter/issues/132620

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

@@ -1956,6 +1956,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
   void dispose() {
     _floatingLabelController.dispose();
     _shakingLabelController.dispose();
+    _borderGap.dispose();
     super.dispose();
   }
 

packages/flutter/test/material/text_form_field_test.dart

@@ -243,6 +243,7 @@ void main() {
     expect(controller.text, ' blah2blah1');
     expect(controller.selection, const TextSelection(baseOffset: 0, extentOffset: 0));
     expect(find.byType(CupertinoButton), findsNothing);
+    controller.dispose();
   },
     variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.linux, TargetPlatform.windows }),
     skip: kIsWeb, // [intended] we don't supply the cut/copy/paste buttons on the web.
@@ -278,11 +279,6 @@ void main() {
     controller.dispose();
   },
     skip: kIsWeb, // [intended] we don't supply the cut/copy/paste buttons on the web.
-    // TODO(polina-c): remove after fixing
-    // https://github.com/flutter/flutter/issues/132620
-    leakTrackingTestConfig:  const LeakTrackingTestConfig(
-      notDisposedAllowList: <String, int?>{'_InputBorderGap' : 1},
-    ),
   );
 
   testWidgets('the desktop cut/copy/paste buttons are disabled for read-only obscured form fields', (WidgetTester tester) async {