reject non-finite values in parse_decimal

[digi-scrypt]

1. parse_decimal builds the result straight from decimal.Decimal(...), which happily accepts nan, inf, Infinity and sNaN, so those parse instead of being rejected.
2. a NaN result then slips past range checks (every comparison against NaN is False), and a signaling NaN throws InvalidOperation the first time anything does arithmetic with it.

Added an is_finite() check so non-finite input raises NumberFormatError like any other non-decimal string. What happens today if someone feeds "inf" from a form field straight into a > limit guard? It passes. Kept it in the parser since that's the layer that already promises to reject things that aren't localized decimals.