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## Problem Druing shard splits we shut down the remote client early and allow the parent shard to keep ingesting data. While ingesting data, the wal receiver task may wait for the current flush to complete in order to apply backpressure. Notifications are delivered via `Timeline::layer_flush_done_tx`. When the remote client was being shut down the flush loop exited whithout delivering a notification. This left `Timeline::wait_flush_completion` hanging indefinitely which blocked the shutdown of the wal receiver task, and, hence, the shard split. ## Summary of Changes Deliver a final notification when the flush loop is shutting down without the timeline cancel cancellation token having fired. I tried writing a test for this, but got stuck in failpoint hell and decided it's not worth it. `test_sharding_autosplit`, which reproduces this reliably in CI, passed with the proposed fix in https://github.com/neondatabase/neon/pull/12304. Closes https://github.com/neondatabase/neon/issues/12060