GHSA-g9hv-x236-4qp3: Russh: client wrong-length X25519 `clone_from_slice` panic (pre-auth DoS)

A malicious SSH server can crash a russh client (an SSH library written in Rust) by sending a malformed key-exchange reply with the wrong length, causing a pre-authentication denial-of-service (an attack that crashes the connection before the server's identity is verified). The bug exists because the client-side code for Curve25519 (a key-exchange algorithm) doesn't validate that the server's ephemeral key is exactly 32 bytes long before trying to copy it into a 32-byte buffer, while the server-side code has this check.