Fix spawn error handling

[cocoa-xu]

Summary

• return structured errors when Unix spawn fails instead of crashing the GenServer call
• preserve module on_exit callbacks supplied at spawn time and align callback specs/docs
• harden Unix NIF read/write/error paths and make native source changes trigger rebuilds
• replace the placeholder test with focused ExPTY behavior coverage

Tests

• mix format --check-formatted
• mix compile --warnings-as-errors
• mix test