syncSearchActive method
Synchronises isSearchOpen with the parent widget's isSearchActive prop.
Called from didUpdateWidget when the prop changes externally so that
isSearchOpen always reflects the actual widget state.
Implementation
void syncSearchActive(bool isActive) {
if (_isSearchOpen == isActive) return;
_isSearchOpen = isActive;
// No notifyListeners — the parent widget is already handling the rebuild.
}