Bug: Scheduler stops running jobs when DST ends (clocks fall back)

[srics96]

Bug Description

When Daylight Saving Time ends and clocks fall back by one hour, jobs scheduled with timezone-aware .at() times can stop running entirely. The _schedule_next_run method calculates the next run time incorrectly during the DST transition, causing next_run to be set to a time that never arrives or gets stuck in a loop.

Steps to Reproduce

import schedule

# Schedule a job at a specific time in a DST-observing timezone
schedule.every().day.at("02:30", "Europe/Berlin").do(my_job)

# When clocks fall back from 3:00 to 2:00 on the last Sunday of October,
# the scheduler either:
# 1. Stops scheduling the job entirely
# 2. Schedules it at the wrong time
# 3. Runs it twice (once for each occurrence of 2:30)

Root Cause

The _correct_utc_offset method in Job doesn't properly handle the ambiguous time period during DST fall-back transitions. When a local time occurs twice (e.g., 2:30 AM occurs once in CEST and once in CET), the method may pick the wrong occurrence or fail to advance past the transition.

Fix

The fix should handle the fold attribute on datetime objects to disambiguate times during DST transitions. When computing the next run after a DST fall-back:

• Use datetime.fold = 1 to select the second (post-transition) occurrence
• Ensure _correct_utc_offset properly advances past the transition

References

• Upstream: fix bug when DST ends dbader/schedule#648 (unmerged PR with fix)
• Upstream: Daylight saving to winter time caused scheduler to stop dbader/schedule#636 (user report: "Daylight saving to winter time caused scheduler to stop")
• Upstream: Inconsistent startup behaviour on daily tasks at midnight - Timezone related dbader/schedule#609 (related timezone issue)