test: Use transitionDurationObserver in emoji_reaction_test

Replaces hardcoded transition delays with dynamic duration handling
via transitionDurationObserver.pumpPastTransition(tester). This
ensures test stability when transition timings change.

Author: MdSaifAliMolla

test/widgets/emoji_reaction_test.dart

@@ -434,7 +434,7 @@ void main() {
       // request the message action sheet
       await tester.longPress(find.byType(MessageContent));
       // sheet appears onscreen; default duration of bottom-sheet enter animation
-      await tester.pump(const Duration(milliseconds: 250));
+      await transitionDurationObserver.pumpPastTransition(tester);
 
       await store.handleEvent(RealmEmojiUpdateEvent(id: 1, realmEmoji: {
         '1': eg.realmEmojiItem(emojiCode: '1', emojiName: 'buzzing'),