Close both HTTP/2 proxy channels after backend failures

Http2ProxyBackendHandler currently closes only one side of a proxied connection when a backend write or handler exception fails. The peer channel can remain open until another event closes it. The proposed change closes both sides on these failure paths. This issue accompanies a Matcha study recommendation derived from https://stackoverflow.com/a/42681543.

Start with Http2ProxyBackendHandler and inspect its backend-write and handler-exception failure paths. The change is complete when both sides of the proxied connection close on each path; run the relevant existing HTTP/2 proxy tests or add coverage if the repository provides them.

Written by the indexing model from the issue text.