What does this PR do?

Fixes launchd_restart() to use the drain-aware _graceful_restart_via_sigusr1() helper instead of the ancestor-guarded _request_gateway_self_restart(). Previously, running hermes gateway restart from a fresh shell always fell through to SIGTERM because the shell is not a descendant of the gateway process, bypassing the drain path that cleanly finishes in-flight agent runs.

Related Issue

Fixes #27745

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• hermes_cli/gateway.py: Replace _request_gateway_self_restart(pid) with _graceful_restart_via_sigusr1(pid, drain_timeout) in launchd_restart(), and update the success message to reflect the drain-aware behavior.
• tests/hermes_cli/test_gateway_service.py: Update two launchd restart tests to mock _graceful_restart_via_sigusr1 instead of _request_gateway_self_restart, adjust assertions for the new function signature and output message.

How to Test

1. Run pytest tests/hermes_cli/test_gateway_service.py -k "launchd_restart" -v — all 3 launchd restart tests should pass.
2. On macOS with a running Hermes gateway managed by launchd: hermes gateway restart from a fresh terminal. The gateway should log a drain-aware restart (SIGUSR1) rather than an immediate SIGTERM.
3. Verify that in-flight agent runs complete before the gateway exits (check gateway.log for drain completion messages).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/hermes_cli/test_gateway_service.py -q and all launchd tests pass (systemd tests fail pre-existing on macOS)
• I've added tests for my changes (updated existing tests to match new code path)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — SIGUSR1 is guarded by hasattr(signal, 'SIGUSR1'), returns False on Windows
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: launchd_restart() in hermes_cli/gateway.py (callers: 2 — gateway restart CLI and hermes update flow)
• Analyzed: _graceful_restart_via_sigusr1() (callers: 2 — systemd_restart and now launchd_restart)
• Blast radius: LOW — single function call change, same pattern already used by systemd_restart
• Related patterns: systemd_restart() at line 2645 uses the same _graceful_restart_via_sigusr1(pid, drain_timeout + 5) pattern. The +5 buffer is not needed for launchd because launchctl kickstart handles the relaunch after exit.