Fix Calendar view computing cron planned runs in UTC instead of the Dag's timezone

[simon-hoon]

closes: #71234

CalendarService._calculate_cron_planned_runs() handed croniter a UTC-tagged start_time when computing "planned" (future, not-yet-executed) calendar cells for cron-based timetables. croniter matches cron fields against whatever wall-clock its start_time carries, so for a Dag on a non-UTC default_timezone, planned runs came out shifted by the UTC offset (e.g. a 08:00 Asia/Seoul schedule showed planned cells at 17:00).

This localizes start_time into the timetable's own timezone before constructing croniter, converting results back to UTC — same math as the common branch of CronMixin._get_next. Its _covers_every_hour DST fold-hour workaround is intentionally not replicated, so every-hour crons in a DST timezone can undercount the folded hour's cell by one run, once a year. Historical (already-executed) runs were unaffected, since those come straight from already-correct DagRun rows.

Added a regression test for a non-UTC cron timetable in test_calendar.py.

Verified with uv sync --package apache-airflow-core + pytest on test_calendar.py: 20/20 passing (19 existing + 1 new).

Before/after (local demo, schedule="0 8 * * *", AIRFLOW__CORE__DEFAULT_TIMEZONE=Asia/Seoul): planned cells moved from the 16:00–17:00 row to the 08:00 row, now matching the Success row above.

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