AbstractBaseSession: Use model fields for subclassed cases

In situations where these fields are overridden in custom models, for instance
extending 'session_key`'s `max_length`. Follow a similar style to `AuthBaseUser`.

See also:
- https://docs.djangoproject.com/en/5.0/topics/http/sessions/#extending-database-backed-session-engines
- https://github.com/django/django/blob/5.0.6/django/contrib/sessions/base_session.py
- https://github.com/typeddjango/django-stubs/blob/5.0.0/django-stubs/contrib/sessions/base_session.pyi#L13-L21

django-stubs/contrib/sessions/base_session.pyi

@@ -11,9 +11,9 @@ class BaseSessionManager(models.Manager[_T]):
     def save(self, session_key: str, session_dict: dict[str, Any], expire_date: datetime) -> _T: ...
 
 class AbstractBaseSession(models.Model):
-    expire_date: datetime
-    session_data: str
-    session_key: str
+    session_key: models.CharField(primary_key=True)
+    session_data: models.TextField()
+    expire_date: models.DateTimeField()
     objects: Any
 
     @classmethod