🗒️ Description

One more line went under the radar in modexp.cpp - 512 bit ctz. This prompted me to generalize the test cases for varying moduli width and parity to make it more readable and thorough

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Set appropriate labels for the changes (only maintainers can apply labels).