Fix CanaryEvents generation timing
Previously, the DAG generating canary events did not account for the delay caused by Airflow's scheduling mechanism, resulting in events for 8:02 being generated at 9:35.
This commit updates the logic to use the end of the time interval, so that canary events for 8:02 are now generated at 8:02. However, the current solution isn't perfect, as the Airflow interval "8:02-9:35" still describes the generation of canary events timestamped at 9:35.
While a custom Airflow Timetable could address this issue, it would be more complex to implement.
Bug: T365223