Russh: Missing X25519 zero-point validation in hybrid ML-KEM key exchange

Vulnerability The hybrid ML-KEM 768 + X25519 key exchange implementation in russh/src/kex/hybridmlkem.rs does not validate that the remote peer's X25519 public key is not the zero point all-zero 32-byte value. This allows a remote peer to force the X25519 contribution to the combined shared secre...