Preserve legacy API options in parallel checks

[lin-hongkuan]

Summary

• keep the legacy API's already-initialized plugins/options when it builds the file checker manager
• pass those initialized values into multiprocessing workers so non-fork start methods do not re-parse default CLI options
• fall back to serial checking when the initialized legacy state cannot be pickled for worker startup

Fixes #2011

Tests

• PYTHONPATH=src python -m pytest tests\integration\test_api_legacy.py tests\integration\test_checker.py -q
• PYTHONPATH=src python -m pytest tests\unit\test_checker_manager.py tests\integration\test_checker.py tests\integration\test_api_legacy.py -q
• PYTHONPATH=src python -m flake8 src\flake8\api\legacy.py src\flake8\checker.py src\flake8\main\application.py tests\integration\test_api_legacy.py tests\integration\test_checker.py
• git diff --check

Note: python -m pre_commit run --files ... currently reports an existing mypy error in src\flake8\formatting\_windows_color.py, which is outside this diff. The targeted flake8 lint and regression tests pass against the working tree via PYTHONPATH=src.