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()