BUG/MINOR: proxy: release watcher in various commands on CLI abort

BUG/MINOR: proxy: release watcher in various commands on CLI abort

Watcher has been added to protect several commands in src/proxy.c which
iterate over either the proxies or servers list against a runtime
deletion.

However, no io_release has been defined for such commands. This may
cause issue in case the command has yielded and is aborted before its
full completion, for example on client early disconnect. Related applet
context is freed while still subscribed into a proxy or server instance.
When this instance is deleted, a crash occurs on freed applet context
access.

To prevent this, define a dedicated io_release for the following
commands : "show servers state|conn", "show backend" and "show errors".
Any watcher is detached there.

Reported-by: Claude (ANT-2026-6Q55R82E)
This must be backported up to 3.4.