fix(gateway): preserve venv python in systemd unit

[inside-ziwu]

Summary

• preserve the project venv Python entrypoint when generating the systemd system service unit
• avoid resolving the venv interpreter symlink to the underlying base interpreter

Problem

The generated system service unit could point at the underlying uv-managed interpreter instead of the project venv entrypoint. That can bypass the expected environment and break startup with missing dependencies.

Testing

• generated the systemd unit locally and verified the Python path stays on the project venv entrypoint
• verified the existing home-prefix remapping behavior remains unchanged for other paths

[teknium1]

Already fixed on main via PR #8861 (salvage of #7735 by @akhater). The current code drops .resolve() entirely and uses .expanduser() for lexical-only expansion, preserving venv symlinks. Thanks for the contribution!