Fix isort import order: Move ValidationError to django.core.exceptions

- Change import from django.forms to django.core.exceptions
- Matches the pattern used in other model files (corpuses, documents)
- Fixes pre-commit isort check failure

Co-authored-by: JSIV <[email protected]>

Author: github-actions[bot]

opencontractserver/conversations/models.py

@@ -2,8 +2,8 @@
 
 import django
 from django.contrib.auth import get_user_model
+from django.core.exceptions import ValidationError
 from django.db import models
-from django.forms import ValidationError
 from guardian.models import GroupObjectPermissionBase, UserObjectPermissionBase
 
 from opencontractserver.annotations.models import Annotation