ForeignKey with callable default crashes with TypeError on makemigrations #60
Description
Hello! I'm not sure if this is a mistake I'm making or something that should be addressed in syzygy itself.
This is on Django 5.0.14 and Python 3.11.
When using syzygy.operations.AddField to add a ForeignKey to settings.AUTH_USER_MODEL, a TypeError occurs during migration execution due to the model reference not being fully resolved.
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union
File ".../operations.py", line XYZ, in get_pre_add_field_operation
field.db_default = field.get_default()
...
if isinstance(field_default, field.remote_field.model):
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union
I think that Django allows ForeignKey.to to be a string (e.g., "auth.User" or via settings.AUTH_USER_MODEL) until app loading completes. At migration parse time, field.remote_field.model can still be a string, not a type, which breaks isinstance() checks.
Probable Steps to Reproduce
Define a ForeignKey in a custom AddField operation using to=settings.AUTH_USER_MODEL (or a string "app.Model").
Add logic in the operation that evaluates the field default or uses isinstance(..., field.remote_field.model).
Run migrations — Django crashes before migration execution due to the unresolved model reference.