[Fix]: showDateRangePicker with "large" helpText cutoffs the save button (flutter#146049)

Fix flutter#146039

Pre-launch Checklist

Author: zeqinjie

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

@@ -1685,7 +1685,12 @@ class _CalendarRangePickerDialog extends StatelessWidget {
                   child: Column(
                     crossAxisAlignment: CrossAxisAlignment.start,
                     children: <Widget>[
-                      Text(helpText, style: headlineHelpStyle),
+                      Text(
+                        helpText,
+                        style: headlineHelpStyle,
+                        maxLines: 1,
+                        overflow: TextOverflow.ellipsis,
+                      ),
                       const SizedBox(height: 8),
                       Row(
                         children: <Widget>[

packages/flutter/test/material/date_range_picker_test.dart

@@ -268,6 +268,16 @@ void main() {
     });
   });
 
+  testWidgets('Long helpText does not cutoff the save button', (WidgetTester tester) async {
+    helpText = 'long helpText' * 100;
+    saveText = 'make it so';
+    await preparePicker(tester, (Future<DateTimeRange?> range) async {
+      expect(find.text(helpText!), findsOneWidget);
+      expect(find.text(saveText!), findsOneWidget);
+      expect(tester.takeException(), null);
+    });
+  });
+
   testWidgets('Material3 has sentence case labels', (WidgetTester tester) async {
     await preparePicker(tester, (Future<DateTimeRange?> range) async {
       expect(find.text('Save'), findsOneWidget);