Preserve legacy API options in multiprocessing workers

[Mirochill]

Summary

• preserve the already-configured options namespace when multiprocessing workers need to initialize without inherited _mp state
• keep reparsing argv for plugin discovery, but avoid discarding programmatic legacy-API overrides such as max_line_length
• add coverage for the worker-init fallback path and update the process-pool expectations

References

• Fixes Max Line Length in style guide ignored with python 3.14 when checking multiple files #2011

Validation

• local validation not run for the current revision because this collaboration workspace is intentionally coordination-only
• relying on the repository's GitHub Actions checks for remote validation