FIX: email summary subject order should be consistent (#19469)

plugins/chat/lib/extensions/user_notifications_extension.rb

@@ -63,7 +63,9 @@ module Chat::UserNotificationsExtension
     all_channels = grouped_messages.keys
     grouped_channels = all_channels.partition { |c| !c.direct_message_channel? }
     channels = grouped_channels.first
-    dm_users = grouped_channels.last.flat_map { |c| grouped_messages[c].map(&:user) }.uniq
+
+    dm_messages = grouped_channels.last.flat_map { |c| grouped_messages[c] }
+    dm_users = dm_messages.sort_by(&:created_at).uniq { |m| m.user_id }.map(&:user)
 
     # Prioritize messages from regular channels over direct messages
     if channels.any?