fix: isolate race condition test by filtering jobs for specific conversation

The test was checking total ConversationReplyEmailWorker.jobs.size which accumulates
across the entire test suite in CI, causing false failures. Now filters jobs by
conversation_id to only count workers for the specific conversation under test.

spec/services/messages/send_email_notification_service_spec.rb

@@ -74,7 +74,8 @@ describe Messages::SendEmailNotificationService do
         threads.each(&:join)
 
         # Only ONE worker should be scheduled despite 5 concurrent attempts
-        expect(ConversationReplyEmailWorker.jobs.size).to eq(1)
+        jobs_for_conversation = ConversationReplyEmailWorker.jobs.select { |job| job['args'].first == conversation.id }
+        expect(jobs_for_conversation.size).to eq(1)
       end
     end