fix(e2e): add ResponseNotFound retry to statistics test

The test_statistics_endpoint was flaky because it failed immediately
on ResponseNotFound (404) instead of retrying. This happens when the
split-descriptive-statistics worker job hasn't completed yet.

This follows the established pattern used in test_55_first_rows.py,
test_52_search.py, test_53_filter.py, and test_54_rows.py which all
use should_retry_x_error_codes=["ResponseNotFound"] for the same reason.

Author: The-Obstacle-Is-The-Way

e2e/tests/test_14_statistics.py

@@ -11,6 +11,8 @@ def test_statistics_endpoint(normal_user_public_jsonl_dataset: str) -> None:
         relative_url=f"/statistics?dataset={dataset}&config={config}&split={split}",
         check_x_revision=True,
         dataset=dataset,
+        should_retry_x_error_codes=["ResponseNotFound"],
+        # ^ I had 404 errors without it. It should return something else at one point.
     )
 
     content = statistics_response.json()