fix: extract zip documents without external command

[deepanshupal]

Summary

• replace the external unzip subprocess with Python's cross-platform zipfile module
• preserve recursive extracted-file discovery
• add a focused regression for nested zip contents

Fixes #617.

Testing

• .venv/bin/python -m pytest tests/test_document_toolkit.py -q (1 passed)
• Ruff check and format check
• py_compile and diff check