parse_time: handle 12 AM and 12 PM correctly on the 12-hour clock

[simpleqt]

Summary

parse_time adds a flat +12 offset whenever the format contains a period marker and the input contains pm, which breaks both 12 o'clock cases:

>>> from babel.dates import parse_time
>>> parse_time('12:00 PM', locale='en_US')
ValueError: hour must be in 0..23, not 24      # noon crashes

>>> parse_time('12:30 AM', locale='en_US')
datetime.time(12, 30)                          # midnight parsed as 12:30 PM-side

en_US's own medium time format is h:mm:ss a, so parsing noon in the most common locale crashes and midnight silently lands twelve hours off. This converts the 12-hour clock the way it works: 12 AM is 00:00, 12 PM is 12:00, and only hours 1–11 shift by twelve.

Verification

• Red/green: five new test_parse_time cases (12:00 pm, 12:00 PM, 12:30 am, 12 AM, 12:00:15 pm) fail on master (the ValueError: hour must be in 0..23, not 24 and the wrong midnight) and pass with this change; all pre-existing am/pm cases are unaffected.
• pytest tests/test_dates.py → 1196 passed; the 5 remaining failures are the same CLDR-data-drift failures present on master in my environment (skeleton/1076/1192/892 — unrelated to time parsing).
• ruff check clean.

AI Disclosure

This PR was prepared with the assistance of an AI coding agent.

• Tool(s): ZCode (GLM-based coding agent)
• Used for: finding the 12 o'clock edge pair during the parse sweep, implementing the proper 12-hour conversion, and running the red/green and baseline verification described above.

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