Attempt to fix exchange hash calculation

Attempted fixes:
1. Add \r\n to version strings (reverted - incorrect)
2. Add SSH_MSG_KEXINIT byte to KEXINIT payloads (reverted - payloads already contain it)

Current issue:
- OpenSSH client still rejects SSH_MSG_KEX_ECDH_REPLY
- Client not sending NEWKEYS
- Exchange hash calculation still has subtle differences

Deep analysis completed:
- Analyzed 10 OpenSSH source functions
- Verified mpint encoding, key derivation, MAC calculation all correct
- Still need to find remaining exchange hash component differences

markbase-core/src/ssh_server/kex_exchange.rs

@@ -197,6 +197,7 @@ impl KexExchangeHandler {
         
         let mut hasher = Sha256::new();
         
+        // RFC 4253 Section 7: V_C and V_S are version strings (without \r\n based on testing)
         hasher.update(&(client_version.len() as u32).to_be_bytes());
         hasher.update(client_version.as_bytes());