fix(dataset): row-level OOB fallback prevents silent coercion of malformed timestamps

The whole-column errors="coerce" fallback in _convert_temporal_columns() could
silently coerce malformed date strings to NaT when they appeared in the same
column as an out-of-bounds value. The OOB exception was caught first, then the
fallback coerced everything — violating the invariant that only OOB values are
coerced.

Replaced with a row-level fallback: iterate values, coerce only individual
OutOfBoundsDatetime values to NaT, re-raise on all other exceptions (malformed
strings, parse errors). Added parametrized mixed-column test covering both value
orderings. Cleaned up unused test helpers; broadened pytest.raises exception type
to (ValueError, pd.errors.ParserError) for pandas-version safety.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>