Allow catalog-only zones without file= in --bind-conf mode

Zones without a file= property (e.g. ddns zones) are included in
catalog zone output for secondaries but skipped in domains.conf.
Previously --bind-conf required every zone to have file= set.

main_test.go

@@ -482,7 +482,7 @@ zone.example.com    catalog1, file=data/zones/example.com
 	}
 }
 
-func TestIntegrationCLIBindConfMissingFile(t *testing.T) {
+func TestIntegrationCLIBindConfCatalogOnlyZone(t *testing.T) {
 	binary := filepath.Join(t.TempDir(), "catalog-zone-gen")
 	cmd := exec.Command("go", "build", "-o", binary, ".")
 	cmd.Dir = projectDir(t)
@@ -502,7 +502,7 @@ soa:
 `
 	writeTestFile(t, dir, "catz.yaml", configContent)
 
-	// Zone without file= property
+	// Zone without file= property (catalog-only, e.g. ddns zone)
 	inputContent := "zone.example.org    catalog1\n"
 	writeTestFile(t, dir, "zones.txt", inputContent)
 
@@ -514,8 +514,26 @@ soa:
 		"--bind-conf", bindConfPath,
 		filepath.Join(dir, "zones.txt"))
 	out, err := cmd.CombinedOutput()
-	if err == nil {
-		t.Fatal("expected error when file= is missing with --bind-conf")
+	if err != nil {
+		t.Fatalf("unexpected error: %v\n%s", err, out)
 	}
-	assertContains(t, string(out), "missing file=")
+
+	// domains.conf should exist but not contain the catalog-only zone
+	data, readErr := os.ReadFile(bindConfPath)
+	if readErr != nil {
+		t.Fatalf("failed to read domains.conf: %v", readErr)
+	}
+	got := string(data)
+	assertContains(t, got, "# THIS FILE IS GENERATED BY catalog-zone-gen")
+	if strings.Contains(got, "zone.example.org") {
+		t.Error("catalog-only zone without file= should not appear in domains.conf")
+	}
+
+	// Catalog zone should still be generated
+	catZonePath := filepath.Join(dir, "catalog1.example.com.zone")
+	catData, catErr := os.ReadFile(catZonePath)
+	if catErr != nil {
+		t.Fatalf("catalog zone not written: %v", catErr)
+	}
+	assertContains(t, string(catData), "zone.example.org")
 }