Fix fork safety for singleton SDK

[jdwyah]

Summary

• reset inherited singleton state in forked children with a child-only at-fork hook
• preserve configured options so workers lazily create a fresh SDK instance after fork
• bump the package version to 1.2.2 and add POSIX fork tests

Testing

• poetry run pytest tests/test_forking.py

[ayip8]

thanks

[ayip8]

great