fix: count DoQ/UDP test server call before writing reply

countHandler incremented its call counter after w.WriteMsg, but the DNS
client returns as soon as it receives the reply. A test reading the
counter right after Resolve returned could therefore observe a stale
zero, e.g. Test_Edns0_CacheReply intermittently failing on CI with
"cache not hit, server was called: 0" while passing on retry.

Increment the counter before writing the reply so it is guaranteed
visible once the client has the response. Verified by widening the
post-write window to reproduce the failure deterministically, then
confirming the reordered handler passes 500x and under -race.

resolver_test.go

@@ -383,6 +383,11 @@ func nonSuccessHandlerWithRcode(rcode int) dns.HandlerFunc {
 
 func countHandler(call *atomic.Int64) dns.HandlerFunc {
 	return func(w dns.ResponseWriter, msg *dns.Msg) {
+		// Count the call before writing the reply. The client returns as soon
+		// as it receives the response, so a caller that reads this counter right
+		// after Resolve returns would race an increment done after WriteMsg and
+		// could observe a stale zero.
+		call.Add(1)
 		m := new(dns.Msg)
 		m.SetRcode(msg, dns.RcodeSuccess)
 		if cookie := getEdns0Cookie(msg.IsEdns0()); cookie != nil {
@@ -395,7 +400,6 @@ func countHandler(call *atomic.Int64) dns.HandlerFunc {
 			m.IsEdns0().Option = append(m.IsEdns0().Option, cookieOption)
 		}
 		w.WriteMsg(m)
-		call.Add(1)
 	}
 }