Failing test: with the only processing slot held, a request for a new
width of a cached image waits for the slot while the cached file is
rewritten; the answer must come from the rewritten file, so the request
read none of the source before it had a slot. Also a test that a fetch
whose request context ends while it waits for a connection shared by all
hosts gives its host's slot back; that one passes already.
Model: opus-5-5