Add _wait_for_gateway_exit() that polls get_running_pid() to confirm
the old gateway process has actually exited before starting a new one.
If the process doesn't exit within 5s, sends SIGKILL to the specific
PID. Uses the saved PID from gateway.pid (not launchd labels) so it
works correctly with multiple gateway instances under separate
HERMES_HOME directories.
Applied to both launchd_restart() and the manual restart path (replaces
the blind time.sleep(2)).
Inspired by PR #1881 by @AzothZephyr (race condition diagnosis).
Adds 4 tests.
ace2cc6257