fix graceful shutdown exit code to be 0
main() now catches errors from runServer/runServicesOnly and returns
cleanly (exit 0) if shutdown_requested is true. this ensures parity
with Python Prefect server behavior.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>