Critical fix: KEXINIT exchange hash encoding (prepend SSH_MSG_KEXINIT byte)

OpenSSH kexgex.c source code analysis:
- KEXINIT payload stored without SSH_MSG_KEXINIT type byte
- Exchange hash prepends SSH_MSG_KEXINIT byte (20) with adjusted length

Before fix:
- client_kexinit_payload included SSH_MSG_KEXINIT byte
- Direct use without prepending

After fix:
- Remove SSH_MSG_KEXINIT byte from payload
- Prepend byte (20) in exchange hash with length+1
- Both kex_exchange.rs and kex_complete.rs updated

Testing result: MAC still fails, indicating additional encoding issues
Next: Detailed comparison of all exchange hash components

markbase-core/src/ssh_server/kex_complete.rs

@@ -71,11 +71,19 @@ impl KexState {
         // V_S: 服务器版本字符串（SSH string格式）
         write_ssh_string_to_hash(&mut hasher, &self.server_version)?;
         
-        // I_C: 客户端KEXINIT payload（SSH string格式）
-        write_ssh_string_to_hash(&mut hasher, &String::from_utf8_lossy(&self.client_kexinit_payload))?;
+        // OpenSSH kexgex.c: "kexinit messages: fake header: len+SSH2_MSG_KEXINIT"
+        // Remove SSH_MSG_KEXINIT type byte from payloads and prepend it in exchange hash
         
-        // I_S: 服务器KEXINIT payload（SSH string格式）
-        write_ssh_string_to_hash(&mut hasher, &String::from_utf8_lossy(&self.server_kexinit_payload))?;
+        let client_kexinit_without_type = &self.client_kexinit_payload[1..];
+        let server_kexinit_without_type = &self.server_kexinit_payload[1..];
+        
+        hasher.update(&((client_kexinit_without_type.len() + 1) as u32).to_be_bytes());
+        hasher.update(&[20]);  // SSH_MSG_KEXINIT type byte
+        hasher.update(client_kexinit_without_type);
+        
+        hasher.update(&((server_kexinit_without_type.len() + 1) as u32).to_be_bytes());
+        hasher.update(&[20]);  // SSH_MSG_KEXINIT type byte
+        hasher.update(server_kexinit_without_type);
         
         // K_S: 服务器主机密钥blob（SSH string格式）
         hasher.update(server_host_key_blob);