toggleNodeVisibility method
Toggles visibility of a specific node.
Returns the new visibility state, or null if node doesn't exist.
Implementation
bool? toggleNodeVisibility(String nodeId) {
final node = _nodes[nodeId];
if (node == null) return null;
final newVisibility = !node.isVisible;
runInAction(() {
node.isVisible = newVisibility;
});
return newVisibility;
}