Cannot modify GroupOfStopPlaces due to child-stops from MM being members instead of MM point

[assadriaz]

Description:

I am unable to make any changes to this stop place group:
https://stoppested.dev.entur.org/group/NSR:GroupOfStopPlaces:31

The issue appears to be that the group contains members that are child-stops belonging to a MM (MultiModal stop), rather than the MM point itself.
The correct structure should be that the MM point is the member, not its individual child-stops.

Steps to reproduce:

Create a MM stop without checking whether its children are already members in a GroupOfStopPlaces (GOSP).
Attempt to edit the GroupOfStopPlaces that contains those child-stops.
Editing is blocked.
Expected behavior:
When creating a MM stop, the system should verify if any of its children are members of a GOSP and replace them with the MM point as the member.
Actual behavior:
The MM stop is created with children that are already members of a GOSP, causing edit restrictions on that group.

Proposed fix:

Update MM stop creation logic to check if any child-stops are members of a GOSP.
If so, replace those child members with the MM point.