add verify

Author: XJIE6

ydb/core/tx/columnshard/columnshard__write.cpp

@@ -429,6 +429,7 @@ void TColumnShard::Handle(NEvents::TDataEvents::TEvWrite::TPtr& ev, const TActor
         auto result = NEvents::TDataEvents::TEvWriteResult::BuildError(
             TabletID(), 0, NKikimrDataEvents::TEvWriteResult::STATUS_BAD_REQUEST, "only single operation is supported");
         ctx.Send(source, result.release(), 0, cookie);
+        AFL_VERIFY(false)("reason: ", "only single operation is supported");
         return;
     }