FIX: Make chat tracking payload backwards compatible (#20852)

Followup cab4b2cfba,
this was causing client JS errors because the old version
of the client was expecting the old keys, but the new
ruby version of the app was sending different keys via
the MessageBus payload. We can remove this in a couple
of weeks.

plugins/chat/app/services/chat/publisher.rb

@@ -129,7 +129,14 @@ module Chat
     end
 
     def self.publish_user_tracking_state(user, chat_channel_id, chat_message_id)
-      data = { channel_id: chat_channel_id, last_read_message_id: chat_message_id }.merge(
+      data = {
+        channel_id: chat_channel_id,
+        last_read_message_id: chat_message_id,
+        # TODO (martin) Remove old chat_channel_id and chat_message_id keys here once deploys have cycled,
+        # this will prevent JS errors from clients that are looking for the old payload.
+        chat_channel_id: chat_channel_id,
+        chat_message_id: chat_message_id,
+      }.merge(
         Chat::ChannelUnreadsQuery.call(channel_ids: [chat_channel_id], user_id: user.id).first.to_h,
       )