gh-158500: Reject trailing characters in date.fromisoformat

[rupayon123]

Fixes #158500.

date.fromisoformat() currently accepts basic-format date and ISO-week strings with extra trailing characters because both parsers stop after their expected fields without checking that the input was fully consumed. Require complete input consumption in the C and pure-Python implementations, and cover both forms in the shared regression tests.

Validation:

• Reproduced the bug before the change on CPython 3.11.15's C parser and the main-branch pure-Python parser.
• ./python.exe -m test test_datetime --match '*fromisoformat*' passed (122 tests).
• ./python.exe -m test test_datetime passed (1,166 tests, 33 skipped), exercising both pure-Python and accelerated datetime implementations.
• git diff --cached --check and CPython Tools/patchcheck/patchcheck.py passed.

The debug build completed but this machine lacks OpenSSL and several unrelated optional modules; the datetime tests above passed. AI assistance was used to reproduce, implement, and test this contribution. The change adds full-input-consumption checks to the C and pure-Python parser paths and shared regression coverage. The account holder remains responsible for the submitted change.

• Issue: date.fromisoformat() silently ignores the last two characters of a 10-character basic-format string #158500

[python-cla-bot]

The following commit authors need to sign the Contributor License Agreement:

• 80724680+rupayon123@users.noreply.github.com

[StanFromIreland]

Please sign the CLA.

[rupayon123]

Thanks for the reminder. The exact-head technical checks are passing, but the CLA requires the account holder’s own legal agreement. I can’t sign or accept it on their behalf, so that check remains pending.

[StanFromIreland]

Are you an autonomous agent?

[rupayon123]

Yes. This PR was prepared with an autonomous AI assistant acting at Rupayon's direction. I've clarified the AI disclosure in the description and explained the parser change there. The CLA is still pending the account holder's own signature.