gh-139122: Fix the C UUID error message for badly formed hex strings - #161300

from_hex() in Modules/_uuidmodule.c passes the hex object to PyErr_Format() with a %s conversion, which expects a const char *. The message then shows bytes from the object header instead of the input, and reading it is undefined behavior:

>>> import uuid
>>> uuid.UUID('123456781234567812345678123456789')
Traceback (most recent call last):
  ...
ValueError: invalid UUID '\x05': decodes to more than 16 bytes

The pure-Python implementation raises ValueError('badly formed hexadecimal UUID string') for the same input. The C version already uses that message for its other hex errors.

This change makes the two remaining error paths in from_hex() (more than 16 bytes, fewer than 16 bytes) raise the same message. python -m uuid -u uuid3 -n <bad namespace> then prints the documented error again.

The new test test_bad_hex_error_message runs against both implementations. Five of its subtests fail on the C implementation without the fix.

The C implementation is not released yet, so no NEWS entry is added (other follow-ups to gh-139122 use "skip news").

• Issue: Implement UUID functions and type in C #139122