Print-mode assistant-error/aborted exit, RPC pi.shutdown() and stdin-EOF
shutdowns, and the extension command-context shutdown() called
process.exit() before (or racing) session.dispose(), skipping the bounded
browser reaper (releaseTabsForOwner) installed in dispose(). An OMP-owned
Chromium could survive the parent and reparent to PID 1.
Route all four graceful paths through the idempotent, promise-memoized
session.dispose() and await it before the final exit. The RPC
performShutdown no longer emits session_shutdown directly (dispose() emits
it), avoiding a double emit.
Fixes#5643
