Timedelta dag backfill dates don't align with normal schedule

### Body With a timedelta-scheduled dag, the logical dates that you get are relative to the start-date you provide when running the backfill command. **This has been the behavior since 2.x** So, e.g. suppose your dag has schedule of timedelta(days=1). Then, running this ``` airflow dags backfill test_backfill_dag__timedelta --start-date '2025-03-10 03:25:26' --end-date 2025-03-13 --dry-run ``` Will produce these runs: ``` "2025-03-10 03:25:26+00:00" "2025-03-11 03:25:26+00:00" "2025-03-12 03:25:26+00:00" ``` But if the schedule was cron schedule, then it would be what you would expect -- just the "partitions" that are within the range requested. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.