Validate registration retry intervals.

Reject register-only configs that leave the shared retry interval at the
zero value so failed registration attempts cannot spin in a tight loop.
Cover the regression in tests and make the register-only integration test
explicit about its retry interval.

Generated with [Devin](https://cli.devin.ai/docs)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/node-agent/internal/nodeagent/app_integration_test.go

@@ -458,14 +458,15 @@ func TestAppRegistersWithoutControlPlaneTokenWhenUnset(t *testing.T) {
 	exportPath := filepath.Join(t.TempDir(), "export")
 
 	_, stop := startTestApp(t, Config{
-		Port:            "0",
-		ExportPath:      exportPath,
-		MachineID:       "nas-no-token",
-		DisplayName:     "No Token NAS",
-		AgentVersion:    "test-version",
-		ExportLabel:     "register-only",
-		ControlPlaneURL: controlPlane.URL,
-		RegisterEnabled: true,
+		Port:              "0",
+		ExportPath:        exportPath,
+		MachineID:         "nas-no-token",
+		DisplayName:       "No Token NAS",
+		AgentVersion:      "test-version",
+		ExportLabel:       "register-only",
+		ControlPlaneURL:   controlPlane.URL,
+		RegisterEnabled:   true,
+		HeartbeatInterval: time.Second,
 	})
 	defer stop()
 

apps/node-agent/internal/nodeagent/app_test.go

@@ -93,3 +93,22 @@ func TestNewRejectsRegistrationWithoutMachineID(t *testing.T) {
 		t.Fatalf("error = %q, want missing-machine-id message", err.Error())
 	}
 }
+
+func TestNewRejectsRegistrationWithoutHeartbeatInterval(t *testing.T) {
+	t.Parallel()
+
+	_, err := New(Config{
+		ExportPath:      t.TempDir(),
+		ListenAddress:   defaultListenAddress(defaultPort),
+		MachineID:       "nas-1",
+		ControlPlaneURL: "http://127.0.0.1:8081",
+		RegisterEnabled: true,
+	}, log.New(io.Discard, "", 0))
+	if err == nil {
+		t.Fatal("expected missing registration retry interval to fail")
+	}
+
+	if !strings.Contains(err.Error(), "BETTERNAS_NODE_HEARTBEAT_INTERVAL") {
+		t.Fatalf("error = %q, want missing-heartbeat-interval message", err.Error())
+	}
+}

apps/node-agent/internal/nodeagent/config.go

@@ -303,6 +303,10 @@ func validateRuntimeConfig(cfg Config) error {
 		return fmt.Errorf("BETTERNAS_NODE_HEARTBEAT_INTERVAL must be greater than zero")
 	}
 
+	if cfg.RegisterEnabled && cfg.HeartbeatInterval <= 0 {
+		return fmt.Errorf("BETTERNAS_NODE_HEARTBEAT_INTERVAL must be greater than zero when registration is enabled")
+	}
+
 	return nil
 }