Scala webhook example fails with wrong resource cleanup

See the `add-webhook` skill test scenario.

```
Failed to resume worker: WebhookAgent("w1"): Component trapped:
  agent_guest.wasm!alloc::rc::Rc<T,A>::drop_slow
  agent_guest.wasm!rquickjs_core::class::ffi::VTable::finalizer_impl
  agent_guest.wasm!free_gc_object
  ...
  golem:agent/guest@1.5.0#invoke: resource has children
```
