feat: Add a sort option for conversations waiting for a reply from an agent (#7364)

app/models/concerns/sort_handler.rb

@@ -29,5 +29,13 @@ module SortHandler
         )
       )
     end
+
+    def self.sort_on_waiting_since
+      order(
+        Arel::Nodes::SqlLiteral.new(
+          sanitize_sql_for_order('CASE WHEN waiting_since IS NULL THEN now() ELSE waiting_since END ASC, created_at ASC')
+        )
+      )
+    end
   end
 end