Handle OSError during process-worker serialization

[Maxteabag]

Summary

• route OSError raised during payload serialization through the worker error fallback
• retain graceful handling when the fallback cannot be sent over a closed pipe
• add regression coverage proving the client receives an error instead of hanging

Tests

• 1,051 unit tests passed, 2 skipped
• focused process-worker tests and Ruff checks passed

Follow-up to #277.