Fix #471: Enforce correct format for GS1-128 trade measure AIs

[zipus]

This pull request addresses Issue #471 regarding the incorrect parsing of trade measure Application Identifiers (AIs) in the gs1_128 module.

🔍 Problem
The parser was accepting invalid 3-digit AIs such as (310), which are not defined in the GS1 General Specifications. According to the GS1 standard, only AIs in the format (31nn), (32nn), (35nn), and (36nn) are valid for trade measures. For example, (3103) indicates net weight in kilograms with 3 decimal places — (310) is not valid.

✅ Fix

• Updated the parsing logic to strictly enforce the requirement for 4-digit AIs in the mentioned ranges.
• Added doctests to validate both correct and incorrect formats.

📚 Reference
GS1 General Specifications – Release 25.0 (2025), Section 3.6.2 – page 166.

Please let me know if you'd like any adjustments or more test coverage.