DynamoDB: BETWEEN no longer excludes an attribute value of 0

[aryansk]

Fixes #10186.

What

FuncBetween.expr() tested the compared attribute with bare Python truthiness (and attr). Since Decimal("0") is falsy, an item whose attribute value is exactly 0 failed a BETWEEN range even when 0 was inside it — e.g. price BETWEEN 0 AND 100 excluded price = 0 while including price = 5.

Why

The start and end operands already use a zero-safe check (start is not None and (isinstance(start, Decimal) or start)) for exactly this reason; the attribute operand was missed, so 0 behaves differently from any other number.

How

Give the attribute the same zero-safe check in all three branches:

attr_has_value = attr is not None and (isinstance(attr, Decimal) or attr)
if start_has_value and attr_has_value and end_has_value:
    return start <= attr <= end
...
elif start is None and attr_has_value and end_has_value:
    return attr <= end

Tests

New test_between_condition_includes_zero scans with price BETWEEN :lo AND :hi and asserts price = 0 is included (0..100 returns both items; -10..0 returns the zero item). Fails on main ({'five'} == {'five', 'zero'}), passes with the fix. Full test_dynamodb_condition_expressions.py suite passes; ruff findings on comparisons.py are pre-existing and untouched.