DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/lib/extensions/user_email_extension.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Chat::UserEmailExtension
+  def execute(args)
+    super(args)
+
+    if args[:type] == "chat_summary" && args[:memberships_to_update_data].present?
+      args[:memberships_to_update_data].to_a.each do |membership_id, max_unread_mention_id|
+        UserChatChannelMembership.find_by(user: args[:user_id], id: membership_id.to_i)&.update(
+          last_unread_mention_when_emailed_id: max_unread_mention_id.to_i,
+        )
+      end
+    end
+  end
+end