Fix autosummary CLI help typing on Python 3.15

[DavidAGInnovation]

Summary

• ensure translated autosummary CLI help values are plain strings for Python 3.15's argparse typing
• add a regression test covering the parser description, epilog, and action help values

Checks

• pytest -q tests/test_ext_autosummary/test_ext_autosummary.py
• ty check --color=never --python=.venv-ci/bin/python
• Ruff check and format validation for the changed files

[jdillard]

This was fixed upstream in python/cpython#154653 and backported to Python 3.15 in python/cpython#154946, so the Sphinx workaround is no longer needed. Thanks for the contribution!