ssh: Don't reply to PING in preauth phase or during KEX

Obtained from: OpenSSH 5e07dee272c3
Security: CVE-2025-26466
Security: FreeBSD-SA-25:05.openssh
Approved by: so
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 8a16d0831e70530b2fbd682e748bd051de35f192)
(cherry picked from commit 34798cb576bbd2064ab8da372112482bf8e2a7e6)