has_children computation: stress test on nodes with many children

[philipperolet]

Following #9947

When computing has_children with multiple nodes, if a node has lots of children (e.g. the Qonto "17K children" for a notion DB), does it impact the query? does it prevent computing other parents because of the size parameter?

[philipperolet]

After investigation, the computation is correct; the
However the aggregation should be a count rather than a list of all children, more efficient for nodes with many children
AFAIU aggregating directly a boolean won't improve performance and won't necessarily be simpler (i'm 90% on this, so good enough to skip & come back if needed)
We can take the opportunity to change to children_count rather than has_children

[philipperolet]

After further investigation, code already performs a count
Further evidence that trying to aggregate a boolean would not be worth it: https://dust.tt/w/0ec9852c2f/assistant/fJ81UiIrNv
Will change the field to a children_count and close

[philipperolet]

Closed by #10382