fix(ssh): Re-add uint32 prefix for shared secret K in exchange hash and key derivation

OpenSSH sshbuf_put_bignum2_bytes() writes uint32(len) + mpint_data
to the buffer (confirmed from sshbuf-getput-basic.c line 569). Both
kex_gen_hash() via sshbuf_putb() and kex_derive_keys() via
ssh_digest_update_buffer() consume the full buffer including the uint32
prefix.

Fixes 'incorrect signature' error on OpenSSH 10.2.

markbase-core/src/ssh_server/cipher.rs

@@ -534,8 +534,12 @@ impl EncryptedPacket {
                     .ok_or_else(|| anyhow!("cipher_ctos not initialized"))?
             };
 
+            let plaintext_bytes = plaintext_packet.ptr().to_vec();
+            info!("Plaintext packet FULL ({} bytes): {:?}", plaintext_bytes.len(), plaintext_bytes);
             let mut encrypted_packet = plaintext_packet.into_vec();
             cipher.apply_keystream(&mut encrypted_packet);
+            info!("Encrypted packet FULL ({} bytes): {:?}", encrypted_packet.len(), encrypted_packet);
+            info!("MAC FULL ({} bytes): {:?}", mac.len(), mac);
 
             // 更新sequence number
             if is_server_to_client {