Remove unused Asset import and fix test fixtures for new annotation

The annotation on AssetAndTimeSchedule.__init__ was migrated to
Collection[SerializedAsset] | SerializedAssetBase, leaving the
TYPE_CHECKING-only Asset import unreferenced and breaking mypy on
two test files that passed SDK Assets to the core class. The
scheduler tests now import AssetAndTimeSchedule (and the matching
CronTriggerTimetable) from airflow.sdk so the types align with how
DAG authors use it; the timetable test fixture mirrors
core_asset_timetable by constructing SerializedAsset directly.

Author: nailo2c

airflow-core/src/airflow/timetables/assets.py

@@ -32,7 +32,6 @@
 
     import pendulum
 
-    from airflow.sdk.definitions.asset import Asset
     from airflow.timetables.base import DagRunInfo, DataInterval, TimeRestriction
 
 

airflow-core/tests/unit/jobs/test_scheduler_job.py

@@ -90,8 +90,10 @@
     DAG,
     Asset,
     AssetAlias,
+    AssetAndTimeSchedule,
     AssetWatcher,
     CronPartitionTimetable,
+    CronTriggerTimetable,
     IdentityMapper,
     StartOfHourMapper,
     task,
@@ -100,9 +102,7 @@
 from airflow.sdk.definitions.timetables.assets import PartitionedAssetTimetable
 from airflow.serialization.definitions.dag import SerializedDAG
 from airflow.serialization.serialized_objects import LazyDeserializedDAG
-from airflow.timetables.assets import AssetAndTimeSchedule
 from airflow.timetables.base import DagRunInfo, DataInterval
-from airflow.timetables.trigger import CronTriggerTimetable
 from airflow.utils.session import create_session, provide_session
 from airflow.utils.sqlalchemy import with_row_locks
 from airflow.utils.state import CallbackState, DagRunState, State, TaskInstanceState

airflow-core/tests/unit/timetables/test_assets_timetable.py

@@ -137,16 +137,11 @@ def sdk_asset_and_time_timetable(test_timetable, test_assets) -> SdkAssetAndTime
 
 
 @pytest.fixture
-def core_asset_and_time_timetable(
-    test_timetable: MockTimetable, test_assets: list[Asset]
-) -> CoreAssetAndTimeSchedule:
-    """
-    Pytest fixture for creating an AssetAndTimeSchedule object.
-
-    :param test_timetable: The test timetable instance.
-    :param test_assets: A list of Asset instances.
-    """
-    return CoreAssetAndTimeSchedule(timetable=test_timetable, assets=test_assets)
+def core_asset_and_time_timetable(test_timetable: MockTimetable) -> CoreAssetAndTimeSchedule:
+    return CoreAssetAndTimeSchedule(
+        timetable=test_timetable,
+        assets=SerializedAssetAll([SerializedAsset("test_asset", "test://asset/", "asset", {}, [])]),
+    )
 
 
 @pytest.fixture