Fix manual runs getting a zero-length data interval across DST

[harjothkhara]

Why

On the two days a year the clocks change, a manually triggered run can get a data interval that is zero length, or that ends after the run time.

With CronDataIntervalTimetable("0 2 * * *", timezone="America/New_York"), a manual run at 2024-03-10 07:30Z:

manual    : 2024-03-10 07:00Z -> 2024-03-10 07:00Z   (zero length)
scheduled : 2024-03-09 07:00Z -> 2024-03-10 07:00Z   (correct)

croniter steps back on naive wall-clock time, but make_aware moves a tick that falls in the spring-forward gap onto the next real instant. When the previous tick is in the gap it lands on the current time, so _get_prev returns the time it was asked about.

That zero-length interval then costs a run. next_dagrun_info reads it as a point-in-time run and skips a period (#66132, correct for the case it was written for):

after the bad interval : 2024-03-11 06:00Z -> 2024-03-12 06:00Z
if the interval is ok  : 2024-03-10 07:00Z -> 2024-03-11 06:00Z

The fall-back side overshoots instead. With 0 1 * * * in America/New_York at 2024-11-03 05:30Z, _get_prev returns 06:00Z — half an hour in the future — so CronTriggerTimetable with run_immediately=True fires a tick that has not happened yet.

Also reproduces in Europe/Zurich, Australia/Sydney and Australia/Lord_Howe.

What

_get_prev now keeps stepping cron.get_prev until the result is strictly earlier than the time asked about. get_prev is strictly decreasing on naive time, so it terminates.

The _covers_every_hour branch is untouched — it uses a wall-clock delta rather than make_aware, so it cannot hit this.

Only values that were already wrong change. Nothing outside a DST transition window is affected.

Testing

Three regression tests, all failing before and passing after:

• test_cron_infer_manual_data_interval_dst — gap and fall-back, including Australia/Lord_Howe for its 30-minute offset
• test_cron_next_dagrun_info_after_dst_manual_run — the skipped period
• test_cron_trigger_run_immediately_does_not_pick_future_run — CronTriggerTimetable, which shares the mixin

airflow-core/tests/unit/timetables/ is 246 passed.

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