CVE-2026-46705: russh server userauth state is not reset when authentication principal changes

The russh server authentication path keeps internal userauth state across SSH_MSG_USERAUTH_REQUEST messages without separating that state when the request principal changes.

RFC 4252 allows the user name and service name fields to change between authentication requests. The issue is not that such changes are invalid. The issue is that russh-owned authentication state, such as remaining methods, partial-success state, and in-progress method state, can remain associated with the connection and then influence a later request for a different (user, service).

This is an internal library state mismatch. Applications are responsible for any authentication state they keep in their own handlers, but russh must reset or separate state that russh itself owns.