Use relative import for _test_helpers (CI fix)

The absolute 'from tests.durabletask...' import worked locally because pip install -e .[dev] adds the repo root to sys.path, but CI runs pytest without that and pytest's rootdir doesn't include a 'tests' top-level package. Switch to a relative import inside the history_export test package (which already has __init__.py).

Author: andystaples

tests/durabletask/extensions/history_export/test_client.py

@@ -34,7 +34,7 @@
 from durabletask.extensions.history_export.activities import clear_context
 from durabletask.testing import create_test_backend
 
-from tests.durabletask.extensions.history_export._test_helpers import wait_until
+from ._test_helpers import wait_until
 
 
 PORT = 50263

tests/durabletask/extensions/history_export/test_entity.py

@@ -30,7 +30,7 @@
 )
 from durabletask.testing import create_test_backend
 
-from tests.durabletask.extensions.history_export._test_helpers import wait_until
+from ._test_helpers import wait_until
 
 
 PORT = 50260

tests/durabletask/extensions/history_export/test_orchestrator.py

@@ -32,7 +32,7 @@
 from durabletask.extensions.history_export import orchestrator as orch_mod
 from durabletask.testing import create_test_backend
 
-from tests.durabletask.extensions.history_export._test_helpers import wait_until
+from ._test_helpers import wait_until
 
 
 PORT = 50262