DEV: restore AutoJoinChannelBatch job as a no-op to clear queue (#29743)

The job was removed in 6dfe2fbe16a4096ff342ec8b626ac62a19eaa180 as part of a performance-related refactor.

The issue was that job was already enqueued in sidekiq and now that the file has been deleted, it's generating a lot of `uninitialized constant Jobs::Chat::AutoJoinChannelBatch` errors.

Restoring this file will help clear the sidekiq queue. We'll remove the job in a few months.

Internal ref - t/141563

plugins/chat/app/jobs/regular/chat/auto_join_channel_batch.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# TODO: delete this unused job after 2025-01-01
+
+module Jobs
+  module Chat
+    class AutoJoinChannelBatch < ::Jobs::Base
+      def execute(args)
+        # no-op
+      end
+    end
+  end
+end