fix(python): Use Decimal for DecimalValue to preserve ISO 19848 precision

[ronan-fesselier]

Good day,

The Python SDK currently stores ISO 19848 Decimal values as Python float, which is a conformance gap with the standard and other SDKs:

• ISO 19848 Table 2 and iso19848-v2024-format-data-types.json.gz define Decimal as an exact value "representable as i / 10^n".
• The DataChannelList XSD types Range/Restriction bounds as xs:decimal, confirming decimal precision is expected for these values.

Python float is a binary floating-point type (IEEE 754 double), which reintroduces rounding errors that the Decimal type is designed to avoid.

This PR replaces float storage with decimal.Decimal from the Python standard library.

Regards,