contains method
Implementation
@override
bool contains(DiagramPoint point) {
if (!bounds.contains(point)) return false;
final radius = effectiveCornerRadius;
if (radius == 0) return true;
final nearest = DiagramPoint(
point.x.clamp(bounds.left + radius, bounds.right - radius),
point.y.clamp(bounds.top + radius, bounds.bottom - radius),
);
final delta = point - nearest;
return delta.x * delta.x + delta.y * delta.y <= radius * radius;
}