Corrects a lint error in the Gemini sample (#2968)

* Linting Gemini example

* Ran the sample update workflow.

Author: redbrogdon

pkgs/dartpad_ui/lib/samples.g.dart

@@ -300,7 +300,7 @@ class MessageWidget extends StatelessWidget {
             decoration: BoxDecoration(
               color: isFromUser
                   ? Theme.of(context).colorScheme.primaryContainer
-                  : Theme.of(context).colorScheme.surfaceVariant,
+                  : Theme.of(context).colorScheme.surfaceContainerHighest,
               borderRadius: BorderRadius.circular(18),
             ),
             padding: const EdgeInsets.symmetric(