CVE-2026-116794: russh: Agent server never calls `Agent::confirm`, so keys added with the confirm constraint (`ssh-add -c`) sign without confirmation

russh is a Rust SSH client and server library that includes an SSH agent protocol implementation. From 0.50.0-beta.12 until 0.65.0, the agent server's Connection::trysign path in russh/src/keys/agent/server.rs recognizes SSHAGENTCONSTRAINCONFIRM but returns a hard-coded successful result instead of calling Agent::confirm. In applications that override Agent::confirm to require user approval or another policy check, a process that can reach the agent socket, including the remote side of a forwarded agent, can use a key added through ssh-add -c without that check and can authenticate or sign data as the user while the key remains loaded. The separate Agent::confirmrequest hook still runs for every signing request, so checks implemented there remain effective, and implementations that retain the default always-successful Agent::confirm do not lose an approval check through this regression. This issue is fixed in version 0.65.0.