Shutting down no longer stalls for 30 seconds behind a closing background search

Code completion keeps a small background search running to learn which names your code actually uses, so suggestions can be ranked by likelihood instead of alphabetically. If the portal began shutting down at the very moment that search was wrapping up, the two could end up waiting on each other: shutdown waited its full 30-second budget for the search to stop, then reported that work had been left running — and the search itself was never told that its owner had gone away. Both halves are fixed: a closing search can no longer hold up shutdown, and the search is stopped with the component that started it, so shutting down is once again as quick as it should be.