fix: last_activity_at is nil when conv is created (#9934)

The payload does not include last_activity_at when the conversation is created. Because of this the frontend is not able to sort the conversations when appending this. Another problem is that the last_activity_at is not always present, it is added only when a message is created, and it updates it. So this can be nil when the conversation is created, so we fallback to created_at only at the presentation layer

app/models/conversation.rb

@@ -124,6 +124,10 @@ class Conversation < ApplicationRecord
     last_message_in_messaging_window?(messaging_window)
   end
 
+  def last_activity_at
+    self[:last_activity_at] || created_at
+  end
+
   def last_incoming_message
     messages&.incoming&.last
   end