Fix showDateRangePicker is missing dartpad tag and cleanup (#144475)

fixes [`showDateRangePicker` interactive sample not loading](https://github.com/flutter/flutter/issues/144474)

### Description 

This fixes `showDateRangePicker` example not loading in Dartpad.

Similar fix was made for `showDatePicker` in https://github.com/flutter/flutter/pull/99401

examples/api/lib/material/date_picker/show_date_picker.0.dart

@@ -13,10 +13,9 @@ class DatePickerApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
+    return const MaterialApp(
       restorationScopeId: 'app',
-      home: const DatePickerExample(restorationId: 'main'),
+      home: DatePickerExample(restorationId: 'main'),
     );
   }
 }

examples/api/lib/material/date_picker/show_date_range_picker.0.dart

@@ -13,10 +13,9 @@ class DatePickerApp extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      theme: ThemeData(useMaterial3: true),
+    return const MaterialApp(
       restorationScopeId: 'app',
-      home: const DatePickerExample(restorationId: 'main'),
+      home: DatePickerExample(restorationId: 'main'),
     );
   }
 }

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

@@ -1020,7 +1020,7 @@ class _DatePickerHeader extends StatelessWidget {
 ///
 /// {@macro flutter.widgets.RestorationManager}
 ///
-/// {@tool sample}
+/// {@tool dartpad}
 /// This sample demonstrates how to create a restorable Material date range picker.
 /// This is accomplished by enabling state restoration by specifying
 /// [MaterialApp.restorationScopeId] and using [Navigator.restorablePush] to