fix(factory): compose relationship creation

[abdusco]

Problem

Creating an optional direct parent relationship through a factory currently writes the child once, creates the parent, then writes the child again to set its foreign key. The final data is correct, but the second write is unnecessary and triggers observable update side effects.

This is not the best behavior for temporal or versioned tables, audit triggers, and any other database feature that reacts to updates.

Minimal reproduction

Generate factories for these two tables:

CREATE TABLE sponsors (
    id INTEGER PRIMARY KEY
);

CREATE TABLE videos (
    id INTEGER PRIMARY KEY,
    sponsor_id INTEGER UNIQUE REFERENCES sponsors(id)
);

Create an update-audit trigger on videos, then create a video with a new sponsor:

video := factory.New().NewVideo(
    factory.VideoMods.WithNewSponsor(),
).CreateOrFail(ctx, t, db)

Expected: the returned video includes its sponsor and no videos update occurs.

Actual on main: the video includes its sponsor, but the audit trigger records one UPDATE after both records have been created.

I reproduced this on local PostgreSQL 18.4: upstream main recorded one update; this branch recorded zero.

Fix

Create direct parent and child relationships as part of one creation flow whenever the relationship is represented by the child foreign key. The factory inserts the child with the parent foreign key already set, avoiding the follow-up relationship update. Explicit foreign-key values remain sufficient to reference an existing parent, and cascading parent setup only fills relationships that are required.