fix(factory): prevent stack overflow for cyclic existing relations

[abdusco]

Problem

Factories can accept an existing model as a relationship. If that model has its relationships loaded, and those relationships point back to it, factory construction can follow the circular graph indefinitely.

For example, an existing User containing loaded Videos, where each Video points back to that same User, causes a stack overflow when the user is supplied as an existing relationship for a new video.

Factory APIs should safely accept loaded bidirectional model graphs. They should preserve the existing relationship where needed, but stop following a model that has already been encountered during the same construction.

Fix

Track models encountered while converting existing relationship graphs, regardless of which factory API begins the conversion. Once a model has already been visited, stop traversing through its loaded relationships.

Regression Test

Adds a generated, no-database test using the existing users/videos fixture:

user := &models.User{}
video := &models.Video{}
user.R.Videos = models.VideoSlice{video}
video.R.User = user

_ = factory.New().NewVideo(factory.VideoMods.WithExistingUser(user))

The test builds a minimal in-memory relationship cycle and verifies that supplying the existing user relationship completes rather than crashing the test process.

Fixes #761.