Fix updater restart argv handoff

[xiami762]

Summary

• restore updater restart argv reconstruction for CLI restarts
• add a Windows restart handoff helper that waits for the old backend to exit and frees the backend port before spawning the restart command
• cover restart handoff behavior and Windows updater restart paths with tests

Tests

• .venv/bin/python -m pytest tests/updater/test_restart_handoff.py tests/updater/test_updater.py