Refactor XMLTokener.unescapeEntity() to reduce complexity

Extracted hex and decimal parsing logic into separate methods to
address SonarQube complexity warning:
- parseHexEntity(): handles ઼ format
- parseDecimalEntity(): handles { format

This reduces cyclomatic complexity while maintaining identical
functionality and all validation checks.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: OwenSanzas