refactor: extract debug mode message as named constant

Author: crackedhandle

packages/devtools_app/lib/src/screens/performance/panes/rebuild_stats/rebuild_stats.dart

@@ -20,6 +20,12 @@ import '../../../../shared/ui/common_widgets.dart';
 import '../flutter_frames/flutter_frame_model.dart';
 import 'rebuild_stats_model.dart';
 
+@visibleForTesting
+const debugModeOnlyMessage =
+    'Rebuild information is not available for this frame.\n'
+    'Widget rebuild counts are only available when running '
+    'an app in debug mode.';
+
 class RebuildStatsView extends StatefulWidget {
   const RebuildStatsView({
     super.key,
@@ -97,12 +103,7 @@ class _RebuildStatsViewState extends State<RebuildStatsView>
         false;
     if (isProfileBuild) {
       return const Center(
-        child: Text(
-          'Rebuild information is not available for this frame.\n'
-          'Widget rebuild counts are only available when running '
-          'an app in debug-mode.',
-          textAlign: TextAlign.center,
-        ),
+        child: Text(debugModeOnlyMessage, textAlign: TextAlign.center),
       );
     }
     return Column(