fix: bot handoff should set waiting time (#13417)

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/models/conversation.rb

@@ -159,6 +159,7 @@ class Conversation < ApplicationRecord
   end
 
   def bot_handoff!
+    update(waiting_since: Time.current) if waiting_since.blank?
     open!
     dispatcher_dispatch(CONVERSATION_BOT_HANDOFF)
   end