datetime fold value cleanup on increment

Bug report

Bug description:

Add one microsecond to datetime with fold=1 and this fold is cleanup and we go to the past instead of go to one microsecond ahead

from datetime import datetime, timedelta
from zoneinfo import ZoneInfo

tz = ZoneInfo('Europe/Paris')
dt1 = datetime(2025, 10, 26, 2, 0, 1, tzinfo=tz)
print(dt1)
dt2 = datetime(2025, 10, 26, 2, 0, 1, fold=1, tzinfo=tz)
print(dt2)
dt3 = dt2 + timedelta(microseconds=1)
print(dt3)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

先執行提供的 Python 重現程式，使用 datetime、timedelta 和 ZoneInfo 處理 Europe/Paris，接著檢查相關的 datetime 算術和 fold 行為。完成的標準是：對 fold=1 值加上一微秒後，產生緊接在後的 datetime，而不是移回過去。