Apply first() of relation mapping modifiers and onBuild() hooks per parent (#127)

[lehni]

What

first() in a relation mapping's modify, or in an onBuild() hook of a graph modifier, limits the related models of each parent to one again under the limitPerParent default. LimitPerParentOperation reads first() as a limit of 1 when the query is built, and first() doesn't unwrap the result of a query that is limited per parent.

Why

WhereInEagerOperation turned first() into limit(1) right after the graph modifiers ran. The relation mapping's modify and onBuild() hooks run later, while the query is built, so their first() stayed a plain first(): only one parent of the whole batch got a related model and the others got null. 3.9.0 with limitPerParent: true applied it per parent.

Tests

• should apply first() of a relation mapping modify per parent
• should apply first() of an onBuild() hook per parent

Both fail without the change.

Part of #53.