gh-152157: Reject empty fraction before timezone in C fromisoformat - #152161

Fixes gh-152157

datetime.fromisoformat() and time.fromisoformat() accepted, in the C accelerator, a
decimal separator (. or ,) followed by zero fractional digits when a timezone
designator (+/-/Z) came next, e.g. '2020-01-01T12:34:56.+05:00' or '12:34:56.Z'.
The pure-Python implementation already raised ValueError for these. ISO 8601 §4.2.2.4 and
RFC 3339 §5.6 require at least one digit after the decimal sign, so the C side was the
lenient/incorrect one.

Divergence (before this PR)

After this PR both implementations raise ValueError for all of the above, and all
previously-accepted valid strings (.5, .123456, comma fractions, no-fraction +offset,
Z) parse identically on both.

Fix

In parse_hh_mm_ss_ff() the decimal-separator case is now handled before the generic
end-of-substring check, so a separator with no following digit is rejected instead of being
treated as trailing content. The standalone trailing separator ('12:34:56.') remains
rejected. No change to the valid-fraction or no-fraction paths.

Note for reviewers

The spec requires ≥1 digit after the decimal sign, and the C implementation already rejects
a standalone trailing separator, so this PR fixes C to reject. The alternative direction
(making pure-Python lenient) is possible but contradicts both the spec and C's own
end-of-string behavior; happy to flip the approach if you'd rather standardize on lenient.

Tests

Added the four empty-fraction-before-tz inputs (offset, negative offset, Z, comma) to the
datetime and time fromisoformat failure tests, which run against both the C and
pure-Python implementations. Positive parity (valid fractions, comma fractions, and
no-fraction-with-offset) is already covered by the existing *_examples tests, which also
run on both implementations.

./python.exe -m test test_datetime passes (both impls), with no refleaks for the touched
tests.