test(email-list): drop redundant accountId arg to satisfy --fatal-infos

The CI dart analyze step runs with --fatal-infos so any
avoid_redundant_argument_values info fails the build. The new
EmailThreadListController test passed the default accountId explicitly,
tripping the lint. Use the default instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/widget/email_thread_list_controller_test.dart

@@ -93,7 +93,7 @@ void main() {
     test('multi-account threads are kept independent in the selection', () {
       final ctrl = EmailThreadListController()
         ..updateThreads([
-          _t('a', accountId: 'acc-1'),
+          _t('a'),
           _t('b', accountId: 'acc-2'),
         ]);
       ctrl.selectAll();