Unsubscribe not called for observables that use syncObservable

[msantang78]

Bug: unsubscribe Not Triggered with syncObservable

This issue is related to #621 but occurs when an observable is converted to synchronous using syncObservable.

Description

When using syncObservable, the underlying observable’s unsubscribe callback is not executed after all observers are removed. As a result, the source subscription remains active even though there are no remaining observers.

Expected Behavior

Once the last observer unsubscribes, the underlying observable should trigger its unsubscribe logic.

Actual Behavior

unsubscribe is never called after all observers are removed.

         let numSubscribes = 0;
         let numUnsubscribes = 0;

         const store$ = observable<any>({});

          syncObservable(store$, {
              subscribe: ({ update }) => {
                  console.log('subscribed');
                  numSubscribes++;
                  update({ value: { value: 1 }, mode: 'set' });
                  return () => {
                      console.log('unsubscribed');
                      numUnsubscribes++;
                  };
              },
          });

         const Test = observer(function Test() {
             
             return createElement('div', undefined, store$.value.get());
         });

         const { unmount } = render(<Test />);

         act(() => {
             unmount();
         });

         await waitFor(() => promiseTimeout(0));

         expect(numSubscribes).toBe(1);
         expect(numUnsubscribes).toBe(1);