Graph cycle/family collapsing: children display as jobs

>Fix the issue where collapsed families register their children as jobs (see screenshot).
>
>![Image](https://github.com/user-attachments/assets/c4a0b030-b6e4-4ca5-b93f-a6acb0f583ee)

Moved from https://github.com/cylc/cylc-ui/issues/1130