test(mobile): preserve full-key mention accessibility

Signed-off-by: Logan Johnson <loganj@squareup.com>

mobile/test/features/channels/message_content_exact_mentions_test.dart

@@ -6,6 +6,7 @@ void main() {
   testWidgets('namesake chips resolve exact tagged keys, not tag order', (
     tester,
   ) async {
+    final semantics = tester.ensureSemantics();
     final first = 'a' * 64;
     final second = 'b' * 64;
     String? tapped;
@@ -23,6 +24,8 @@ void main() {
     expect(tapped, first);
     await tester.tap(find.text('Scout (bbbbbbbb…bbbb)'));
     expect(tapped, second);
+    expect(find.bySemanticsLabel(RegExp('Scout.*$second')), findsOneWidget);
     expect(tester.takeException(), isNull);
+    semantics.dispose();
   });
 }