fix(gateway): kill tool subprocesses before adapter disconnect on drain timeout (#14728)

Closes #8202.

Root cause: stop() reclaimed tool-call bash/sleep children only at the
very end of the shutdown sequence — after a 60s drain, 5s interrupt
grace, and per-adapter disconnect. Under systemd (TimeoutStopSec bounded
by drain_timeout), that meant the cgroup SIGKILL escalation fired first,
and systemd reaped the bash/sleep children instead of us.

Fix:
- Extract tool-subprocess cleanup into a local helper
  _kill_tool_subprocesses() in _stop_impl().
- Invoke it eagerly right after _interrupt_running_agents() on the
  drain-timeout path, before adapter disconnect.
- Keep the existing catch-all call at the end for the graceful path
  and defense in depth against mid-teardown respawns.
- Bump generated systemd unit TimeoutStopSec to drain_timeout + 30s
  so cleanup + disconnect + DB close has headroom above the drain
  budget, matching the 'subprocess timeout > TimeoutStopSec + margin'
  rule from the skill.

Tests:
- New: test_gateway_stop_kills_tool_subprocesses_before_adapter_disconnect_on_timeout
  asserts kill_all() runs before disconnect() when drain times out.
- New: test_gateway_stop_kills_tool_subprocesses_on_graceful_path
  guards that the final catch-all still fires when drain succeeds
  (regression guard against accidental removal during refactor).
- Updated: existing systemd unit generator tests expect TimeoutStopSec=90
  (= 60s drain + 30s headroom) with explanatory comment.

hermes_cli/gateway.py

@@ -1469,7 +1469,14 @@ def generate_systemd_unit(system: bool = False, run_as_user: str | None = None)
             path_entries.append(resolved_node_dir)
 
     common_bin_paths = ["/usr/local/sbin", "/usr/local/bin", "/usr/sbin", "/usr/bin", "/sbin", "/bin"]
-    restart_timeout = max(60, int(_get_restart_drain_timeout() or 0))
+    # systemd's TimeoutStopSec must exceed the gateway's drain_timeout so
+    # there's budget left for post-interrupt cleanup (tool subprocess kill,
+    # adapter disconnect, session DB close) before systemd escalates to
+    # SIGKILL on the cgroup — otherwise bash/sleep tool-call children left
+    # by a force-interrupted agent get reaped by systemd instead of us
+    # (#8202). 30s of headroom covers the worst case we've observed.
+    _drain_timeout = int(_get_restart_drain_timeout() or 0)
+    restart_timeout = max(60, _drain_timeout) + 30
 
     if system:
         username, group_name, home_dir = _system_service_identity(run_as_user)