fix: stabilize makemigrations when SITE_ID != 1 (#34787)

Some models in third_party_auth used settings.SITE_ID as a field
default, which caused Django to say migrations were out of sync whenever
settings.SITE_ID happened to be anything other than 1 for any developer:

    Your models in app(s): 'third_party_auth' have changes that are not
    yet reflected in a migration, and so won't be applied. Run
    'manage.py makemigrations' to make new migrations, and then re-run
    'manage.py migrate' to apply them.

This could easily happen if a developer is testing out site
configuration or site-specific theming and ends up with a SITE_ID other
than 1.

The fix, inspired by a StackOverflow answer [1], is to simply create
a wrapper function for the dynamic default value. The wrapper function,
rather than the current value of SITE_ID, will be serialized to the
migraiton file.

This commit includes a migration file, but from a database perspective,
the migration is a no-op.

[1] https://stackoverflow.com/a/12654998

common/djangoapps/third_party_auth/migrations/0013_default_site_id_wrapper_function.py

@@ -0,0 +1,36 @@
+# Generated by Django 4.2.13 on 2024-05-13 19:22
+
+import common.djangoapps.third_party_auth.models
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('sites', '0002_alter_domain_unique'),
+        ('third_party_auth', '0012_alter_ltiproviderconfig_site_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='ltiproviderconfig',
+            name='site',
+            field=models.ForeignKey(default=common.djangoapps.third_party_auth.models._get_site_id_from_settings, help_text='The Site that this provider configuration belongs to.', on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='sites.site'),
+        ),
+        migrations.AlterField(
+            model_name='oauth2providerconfig',
+            name='site',
+            field=models.ForeignKey(default=common.djangoapps.third_party_auth.models._get_site_id_from_settings, help_text='The Site that this provider configuration belongs to.', on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='sites.site'),
+        ),
+        migrations.AlterField(
+            model_name='samlconfiguration',
+            name='site',
+            field=models.ForeignKey(default=common.djangoapps.third_party_auth.models._get_site_id_from_settings, help_text='The Site that this SAML configuration belongs to.', on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='sites.site'),
+        ),
+        migrations.AlterField(
+            model_name='samlproviderconfig',
+            name='site',
+            field=models.ForeignKey(default=common.djangoapps.third_party_auth.models._get_site_id_from_settings, help_text='The Site that this provider configuration belongs to.', on_delete=django.db.models.deletion.CASCADE, related_name='%(class)ss', to='sites.site'),
+        ),
+    ]

common/djangoapps/third_party_auth/models.py

@@ -88,6 +88,24 @@ def __str__(self):
         )
 
 
+def _get_site_id_from_settings() -> int:
+    """
+    Simply return SITE_ID from settings.
+
+    We define this function so the current SITE_ID can be used as the default value on a
+    couple fields in this module. We can't use `settings.SITE_ID` directly in the model class,
+    because then the migration file will contain whatever the value of SITE_ID was when the
+    migration was created. This value is usually 1, but in the event that a developer is
+    running a non-default site, they would get a value like 2 or 3, which will result in
+    a dirty migration state. By defining this wrapper function, the name
+    `_get_site_id_from_settings` will be serialized to the migration file as the default,
+    regardless of what any developer's active SITE_ID is.
+
+    Reference: https://docs.djangoproject.com/en/dev/ref/models/fields/#default
+    """
+    return settings.SITE_ID
+
+
 class ProviderConfig(ConfigurationModel):
     """
     Abstract Base Class for configuring a third_party_auth provider
@@ -143,7 +161,7 @@ class ProviderConfig(ConfigurationModel):
     )
     site = models.ForeignKey(
         Site,
-        default=settings.SITE_ID,
+        default=_get_site_id_from_settings,
         related_name='%(class)ss',
         help_text=_(
             'The Site that this provider configuration belongs to.'
@@ -455,7 +473,7 @@ class SAMLConfiguration(ConfigurationModel):
     KEY_FIELDS = ('site_id', 'slug')
     site = models.ForeignKey(
         Site,
-        default=settings.SITE_ID,
+        default=_get_site_id_from_settings,
         related_name='%(class)ss',
         help_text=_(
             'The Site that this SAML configuration belongs to.'