fix: data race on test DNS handler flag under -race

TestClientAgainstLocalServer captured RecursionDesired into a plain bool
from the miekg server handler goroutine and read it from the test
goroutine; the UDP round-trip gives no happens-before edge, so CI's
-race run flagged it. Use atomic.Bool.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/dns/real_exchange_test.go

@@ -3,6 +3,7 @@ package dns
 import (
 	"context"
 	"net"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -107,9 +108,11 @@ func TestRealExchangeUnreachable(t *testing.T) {
 }
 
 func TestClientAgainstLocalServer(t *testing.T) {
-	var sawRD bool
+	// Written by the server handler goroutine, read by the test goroutine;
+	// the UDP round-trip provides no happens-before edge, so use an atomic.
+	var sawRD atomic.Bool
 	addr := startTestDNSServer(t, "udp", func(w dns.ResponseWriter, r *dns.Msg) {
-		sawRD = r.RecursionDesired
+		sawRD.Store(r.RecursionDesired)
 		aHandler("5.6.7.8")(w, r)
 	})
 
@@ -127,7 +130,7 @@ func TestClientAgainstLocalServer(t *testing.T) {
 	if len(resp.Answer) == 0 {
 		t.Fatal("expected answers")
 	}
-	if sawRD {
+	if sawRD.Load() {
 		t.Error("wire query must have RD=0")
 	}
 }