Add pip install --group dev, prerequisite for deprecating requirements.txt

[hwanseoc]

Summary

• add a PEP 735 dev dependency group mirroring requirements.txt
• retain requirements.txt for existing CI consumers

Testing

• parsed pyproject.toml with Python 3.12 and verified the group matches requirements.txt
• ran pip install --dry-run --group dev with pip 25.1+

@coderabbitai ignore