DNS-over-QUIC: 重试仅覆盖两种错误,Quad9 都不会返回

I forwarded to Quad9 over QUIC and got intermittent SERVFAILs for months. I switched the same server to DoT, to the same two Quad9 addresses, and they stopped completely. I think the difference is one line in the QUIC client's retry check.