fix(gateway): remove stale default pid file on startup

[jackylai2660707]

Summary

• remove stale default gateway.pid files directly when startup detects an invalid/dead PID
• add a regression test covering the self-restart/systemd restart path where the default pid file points at a dead process

Problem

When a gateway self-restart or --replace handoff leaves behind a stale default gateway.pid, get_running_pid() correctly detects that the PID is dead but _cleanup_invalid_pid_path() routes cleanup through remove_pid_file(). That helper only removes pid files owned by the current process, so the stale pid file survives and subsequent startups keep failing with PID file race lost to another gateway instance.

Testing

• /root/.hermes/hermes-agent/venv/bin/pytest tests/gateway/test_status.py -q

[alt-glitch]

Likely duplicate of #14153, #13709, #14002, #13934 — all fix the same stale gateway.pid cleanup issue. Note #14200 was already merged addressing this.

[alt-glitch]

Likely duplicate of #14153. Note #14200 already merged.