Continuation of #3409 — Group DM notifications are not grouped and every message plays a notification sound

Can reproduce CR - Notifications were properly grouped, but each triggered a notification sound individually despite rapid-firing several messages in a row. Three separate trials returned the same results. Version 2026.10.100, Beta 2, Channel Beta, Build 2591, Android 16 (arm64), Device Google Pixel 9 Pro, Locale en-US, Push fcm, Flutter 3.47.6, Dart 3.13.5