New guards prevent resuming an agent whose stop hasn't finished

resumeAgentBackground now guards against resuming an agent whose stop is still in progress. If the agent attempting the resume has itself been stopped and that stop hasn't finished, the resume throws immediately. Separately, if the target agent's previous run was stopped but hasn't fully exited yet, the resume is blocked with a message telling the user to re-run the stop tool or wait for it to exit. This closes a race where an agent could be resumed, or could resume others, mid-teardown.