FIX: N+1 query for chat message serializer on mentions (#21767)

Followup to d4a5b79592,
this introduced an N1 because every message in the list
we had to query users for the mentions and then the user's
status too. Instead we can just include both in Chat::MessagesQuery.

plugins/chat/app/queries/chat/messages_query.rb

@@ -76,10 +76,14 @@ module Chat
           .includes(:uploads)
           .includes(chat_channel: :chatable)
           .includes(:thread)
-          .includes(:chat_mentions)
           .where(chat_channel_id: channel.id)
 
-      query = query.includes(user: :user_status) if SiteSetting.enable_user_status
+      if SiteSetting.enable_user_status
+        query = query.includes(user: :user_status)
+        query = query.includes(chat_mentions: { user: :user_status })
+      else
+        query = query.includes(chat_mentions: :user)
+      end
 
       query
     end

plugins/chat/app/serializers/chat/message_serializer.rb

@@ -27,9 +27,10 @@ module Chat
     has_many :uploads, serializer: ::UploadSerializer, embed: :objects
 
     def mentioned_users
-      User
-        .where(id: object.chat_mentions.pluck(:user_id))
-        .order("users.id ASC")
+      object
+        .chat_mentions
+        .map(&:user)
+        .sort_by(&:id)
         .map { |user| BasicUserWithStatusSerializer.new(user, root: false) }
         .as_json
     end

plugins/chat/spec/models/chat/message_spec.rb

@@ -451,7 +451,7 @@ describe Chat::Message do
       notification = Fabricate(:notification)
       mention_1 = Fabricate(:chat_mention, chat_message: message_1, notification: notification)
 
-      message_1.destroy!
+      message_1.reload.destroy!
 
       expect { mention_1.reload }.to raise_error(ActiveRecord::RecordNotFound)
       expect { notification.reload }.to raise_error(ActiveRecord::RecordNotFound)