'BaseTaskSerializerBulk' object has no attribute 'db_predictions'

**Describe the bug**
label-studio-1 | [2026-04-13 12:31:47,128] [core.utils.common::custom_exception_handler::97] [ERROR] eaa1da07-eb27-453b-b1a0-ad5524e5b24d 'BaseTaskSerializerBulk' object has no attribute 'db_predictions'
label-studio-1 | Traceback (most recent call last):
label-studio-1 | File "/label-studio/.venv/lib/python3.13/site-packages/rest_framework/views.py", line 506, in dispatch
label-studio-1 | response = handler(request, *args, **kwargs)
label-studio-1 | File "/label-studio/label_studio/data_import/api.py", line 759, in post
label-studio-1 | return super(ReImportAPI, self).post(*args, **kwargs)
label-studio-1 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
label-studio-1 | File "/label-studio/.venv/lib/python3.13/site-packages/django/utils/decorators.py", line 48, in _wrapper
label-studio-1 | return bound_method(*args, **kwargs)
label-studio-1 | File "/label-studio/label_studio/data_import/api.py", line 271, in post
label-studio-1 | return super(ImportAPI, self).post(*args, **kwargs)
label-studio-1 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
label-studio-1 | File "/label-studio/.venv/lib/python3.13/site-packages/rest_framework/generics.py", line 194, in post
label-studio-1 | return self.create(request, *args, **kwargs)
label-studio-1 | ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
label-studio-1 | File "/label-studio/label_studio/core/utils/common.py", line 353, in f_retry
label-studio-1 | return f(*args, **kwargs)
label-studio-1 | File "/label-studio/label_studio/data_import/api.py", line 749, in create
label-studio-1 | return self.sync_reimport(project, file_upload_ids, files_as_tasks_list)
label-studio-1 | ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
label-studio-1 | File "/label-studio/label_studio/data_import/api.py", line 670, in sync_reimport
label-studio-1 | prediction_count = len(serializer.db_predictions)
label-studio-1 | ^^^^^^^^^^^^^^^^^^^^^^^^^
label-studio-1 | AttributeError: 'BaseTaskSerializerBulk' object has no attribute 'db_predictions'. Did you mean: 'add_predictions'?

**To Reproduce**
Steps to reproduce the behavior:
1. Import label studio JSON with predictions.
2. Label and submit.
3. export as label studio JSON
4. Try to import the file.
5. Boom

It seems like it turns the predictions into something like
"predictions" : [ 22 ],

which makes it blow up