fix(tests): pass the new post_index argument at the remaining call site

`failed_document_ids` is required, so this call did not type-check and the
celery external-dependency suite failed to import.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/external_dependency_unit/celery/test_persona_file_sync.py

@@ -947,6 +947,7 @@ class TestSecondaryPassSkipsTerminalSideEffects:
                 enrichment=enricher,
                 db_session=db_session,
                 index_to_secondary=True,
+                failed_document_ids=set(),
             )
             db_session.refresh(uf)
             assert uf.status == UserFileStatus.INDEXING