Ending a turn right after spawning a subagent fired the bell and a toast
saying the response was finished, while the work continued. The Stop
payload carries background_tasks, so skip the signal when it is not
empty; the next turn end signals once the work is really done.
Interaction requests still signal during background work. A missing
field still signals, so an older client loses nothing. Also drop the
message suffix when it merely restates the label.
Bastien Chanot
0e8018ae7b