gh-152204: Validate date fields in pure-Python date.fromisoformat

[tonghuaroot]

_pydatetime._parse_isoformat_date reads each fixed-width field with int() on a slice, without checking that the slice is exactly N ASCII digits. int() accepts a leading +/-/whitespace and a short string, so several malformed ISO 8601 basic-format dates are silently parsed into a wrong-but-plausible date instead of raising ValueError:

>>> import _pydatetime
>>> _pydatetime.date.fromisoformat('2020+12')
datetime.date(2020, 1, 2)
>>> _pydatetime.date.fromisoformat('+020-06-15')
datetime.date(20, 6, 15)
>>> _pydatetime.date.fromisoformat('2020061')   # 7 chars: day slice reads '1'
datetime.date(2020, 6, 1)
>>> _pydatetime.date.fromisoformat('2020-W2')   # 1-digit week number
datetime.date(2020, 1, 6)

The C accelerator rejects all of these via parse_digits() (which requires the exact field width and digit-only content), so this is a C-vs-pure-Python divergence. The pure-Python path is used when the _datetime C extension is unavailable, and directly via _pydatetime.

This validates each field slice (year / month / day / weekno / weekday) to be exactly N ASCII digits before converting, mirroring the C parse_digits(), and extends datetimetester's test_fromisoformat_fails with the affected inputs (the new cases now reject on both implementations).

Fixes #152204.

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Prepared with AI assistance (Claude Code) and verified by hand against a debug build, against both the C and pure-Python implementations.

• Issue: Pure-Python date.fromisoformat silently mis-parses malformed basic-format dates #152204