Adds AgentRunSupervisor.ShutdownBroadcast() returning the handler's
shutdown channel so the StopAndResume integration test can wait for
graceful-shutdown propagation deterministically instead of sleeping
for a fixed duration. The method is explicitly documented as
test-only and not part of the operational contract.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>
20dbc28398