Workers share the same Source instances as pollers and use them for
fetch(). PollerManager.stop() was closing httpx clients before the
worker pool drained, so in-flight fetches during the shutdown grace
hit a closed client.
- Move source closing out of stop() into a separate close_sources()
- Call close_sources() after _stop_pool() in both serve() and
run_batch()
- Promote aclose() to the Source protocol with no-op defaults for
FS and S3, removing the hasattr duck-typing
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