Agent heartbeats unconditionally set status=running on any write
containing agent_latest_ping, so a ping still in flight when a VM
was deliberately halted could immediately revive its status. Now
VirtualMachinesObserver marks a short grace window when status
transitions into 'halted', and pings inside that window are ignored.
A ping arriving after the window means the shutdown didn't actually
take, so the status is still corrected back to running.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>