Preserve leading fractional zeros in plural operands

[emme1t]

extract_operands() undercounts visible fractional digits for numbers below 1 when the fractional part starts with zeros. For example, Decimal('0.001') currently produces v = 1, w = 1; both operands should be 3 under the CLDR operand definitions.

This affects locale plural selection:

from decimal import Decimal
from babel import Locale

Locale('lv').plural_form(Decimal('0.011'))
# Before: 'zero'
# After:  'one'

The Latvian rule distinguishes v = 2 from v != 2. Decimal.as_tuple().digits omits leading fractional zeros, so the current calculation treats 0.011 like a number with two fractional digits.

Compute v from the decimal exponent and derive w by subtracting the trailing-zero count. Fractional values f and t retain their existing calculation. Tests cover leading zeros, trailing zeros, signed values, decimal zero, a positive exponent, float input, custom plural rules, and the Latvian locale rule.

Validation on Windows / Python 3.13.13 with the repository's CLDR 48.2 import:

• New regression cases before the fix: 10 failed.
• python -m pytest tests/test_plural.py -q: 49 passed.
• python -m pytest -q: 7839 passed, 9 skipped, 2 xfailed.
• Ruff lint and format checks pass for both changed files; git diff --check passes.

AI assistance: Prepared with Codex. The reproducer and tests listed above were executed locally.