run() now defaults to auto_end=True: the runner ends once every root
worker has finished, so single-pipeline bots end naturally when their
pipeline does and tests no longer need an explicit runner.end() call.
Multi-worker bots whose helpers run forever still trigger shutdown via
end() / cancel() from an event handler (typically on transport
disconnect). Hosts that add and remove workers across many sessions can
pass auto_end=False to keep the runner up.
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