feat: Add a priority + created at sort for conversations (#13658)

- Add a new conversation sort option "Priority: Highest first, Created:
Oldest first" that sorts by priority descending (urgent > high > medium
> low > none) with created_at ascending as the tiebreaker

app/models/concerns/sort_handler.rb

@@ -14,6 +14,10 @@ module SortHandler
       order(generate_sql_query("priority #{sort_direction.to_s.upcase} NULLS LAST, last_activity_at DESC"))
     end
 
+    def sort_on_priority_created_at(sort_direction = :desc)
+      order(generate_sql_query("priority #{sort_direction.to_s.upcase} NULLS LAST, created_at ASC"))
+    end
+
     def sort_on_waiting_since(sort_direction = :asc)
       order(generate_sql_query("waiting_since #{sort_direction.to_s.upcase} NULLS LAST, created_at ASC"))
     end