fix: set graceperiod to a real timedelta in upstream sync test

The string '2d' couldn't be parsed by Timedelta.from_json and was
silently coerced to None, triggering ModifyingEnforceTypeWarning
(the assertion happened to pass because enforce_type is disabled
for this codepath and the raw value was preserved). Using a real
timedelta avoids the warning and makes the field type explicit.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: feanil

cms/lib/xblock/test/test_upstream_sync.py

@@ -293,7 +293,7 @@ def test_sync_updates_to_downstream_only_fields(self):
         downstream.attempts_before_showanswer_button = 2
         downstream.due = datetime.datetime(2025, 2, 2, tzinfo=datetime.timezone.utc)  # noqa: UP017
         downstream.force_save_button = True
-        downstream.graceperiod = '2d'
+        downstream.graceperiod = datetime.timedelta(days=2)
         downstream.grading_method = 'last_score'
         downstream.max_attempts = 100
         downstream.show_correctness = 'always'
@@ -321,7 +321,7 @@ def test_sync_updates_to_downstream_only_fields(self):
         assert downstream.attempts_before_showanswer_button == 2
         assert downstream.due == datetime.datetime(2025, 2, 2, tzinfo=datetime.timezone.utc)  # noqa: UP017
         assert downstream.force_save_button
-        assert downstream.graceperiod == '2d'
+        assert downstream.graceperiod == datetime.timedelta(days=2)
         assert downstream.grading_method == 'last_score'
         assert downstream.max_attempts == 100
         assert downstream.show_correctness == 'always'