Error "MongoContent matching query does not exist" during migration

[angonz]

When running forum_migrate_course_from_mongodb_to_mysql --no-toggle all, I got the following stack trace:

Traceback (most recent call last):
  File "/openedx/edx-platform/./manage.py", line 106, in <module>
    execute_from_command_line([sys.argv[0]] + django_args)
  File "/openedx/venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/openedx/venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/openedx/venv/lib/python3.11/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/openedx/venv/lib/python3.11/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/openedx/venv/lib/python3.11/site-packages/forum/management/commands/forum_migrate_course_from_mongodb_to_mysql.py", line 48, in handle
    migrate_read_states(db, course_id)
  File "/openedx/venv/lib/python3.11/site-packages/forum/migration_helpers.py", line 253, in migrate_read_states
    thread_id = MongoContent.objects.get(
                ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/openedx/venv/lib/python3.11/site-packages/django/db/models/manager.py", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/openedx/venv/lib/python3.11/site-packages/django/db/models/query.py", line 637, in get
    raise self.model.DoesNotExist(
forum.backends.mysql.models.MongoContent.DoesNotExist: MongoContent matching query does not exist.

This was caused because one of the threads of the read states does not exist in the MongoContent table.
The exception is raised by the first() function after the find here:

                    mongo_content = MongoContent.objects.filter(
                        mongo_id=thread_id
                    ).first()

[angonz]

PR #178