test(api): reset preview before decoding the second response

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/internal/httpapi/generator_test.go

@@ -69,6 +69,7 @@ func TestGeneratorSettingsAndPreview(t *testing.T) {
 		t.Errorf("hostmaster not normalized: %q", g.DNS.Hostmaster)
 	}
 
+	p = service.ConfigPreview{} // json.Unmarshal keeps fields absent from the new response
 	viewer.mustStatus(200, "GET", "/api/v1/config/preview", nil).json(t, &p)
 	if p.DNSError != nil {
 		t.Fatalf("dns error %+v", p.DNSError)