`close()` refused a client covering a set, which left one with no method to
call: `async with` was the only lifecycle it had. `aclose()` runs that teardown
for a caller that owns the client some other way, whatever it covers, and
nothing to release is not an error, so it is safe before entering and after
closing.
`close()` stays what it is, one connection and nothing else, and says so:
draining the background vacuum and releasing the embedder and reranker are
awaitable.
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