parse_decimal: reject underscore grouping with strict=True

[glaziermag]

parse_decimal('1_000', locale='en_US', strict=True) currently returns Decimal('1000'), although strict mode promises to reject irregularly formatted numbers. The strict-validation branch only runs when the locale's grouping separator is present, so Decimal's underscore syntax bypasses it.

Run against the current default branch:

from babel.numbers import parse_decimal

parse_decimal('1_000', locale='en_US', strict=True)
# Actual: Decimal('1000')
# Expected: NumberFormatError

The documented strict-mode guarantee requires an exception for irregular formatting.

This change sends underscore-containing input through the existing strict validation. Regression tests cover en_US and de, including acceptance of valid grouped and ungrouped input and preservation of permissive parsing.

Validation: clean baseline reproduced twice; full local suite on Python 3.12: 7,833 passed, 7 skipped, 2 xfailed. Ruff and git diff --check pass. The new regression cases also pass after the final lint adjustment.

Prepared with AI assistance.