fix: mentions are not rendered properly in slack (#6762)

lib/integrations/slack/send_on_slack_service.rb

@@ -1,4 +1,5 @@
 class Integrations::Slack::SendOnSlackService < Base::SendOnChannelService
+  include RegexHelper
   pattr_initialize [:message!, :hook!]
 
   def perform
@@ -34,12 +35,16 @@ class Integrations::Slack::SendOnSlackService < Base::SendOnChannelService
   def message_content
     private_indicator = message.private? ? 'private: ' : ''
     if conversation.identifier.present?
-      "#{private_indicator}#{message.content}"
+      "#{private_indicator}#{message_text}"
     else
-      "#{formatted_inbox_name}#{email_subject_line}\n#{message.content}"
+      "#{formatted_inbox_name}#{email_subject_line}\n#{message_text}"
     end
   end
 
+  def message_text
+    message.content.present? ? message.content.gsub(MENTION_REGEX, '\1') : message.content
+  end
+
   def formatted_inbox_name
     "\n*Inbox:* #{message.inbox.name} (#{message.inbox.inbox_type})\n"
   end