Fix SSH Phase 7: SFTP packet SSH string format

SFTP protocol fix completed:
- Add wrap_sftp_packet() helper function
- All SFTP responses now use SSH string format (uint32(length) + packet_type + payload)
- SSH_FXP_VERSION response: 9 bytes ✓
- SSH_FXP_REALPATH response: 71 bytes ✓
- SSH_FXP_NAME, SSH_FXP_DATA, SSH_FXP_HANDLE all wrapped correctly

Test results:
- SSH_FXP_INIT (version 3) → SSH_FXP_VERSION working ✓
- SSH_FXP_REALPATH (path=.) → SSH_FXP_NAME working ✓
- SFTP handshake successful ✓
- Connection established (sftp connects successfully) ✓

SFTP packet format fix:
- SFTP packets need SSH string format: uint32(length) + packet_type + payload
- SSH_MSG_CHANNEL_DATA adds another SSH string layer (double wrapping)
- Client expects: [length, packet_type, payload] format

Files modified:
- sftp_handler.rs: Added wrap_sftp_packet() and wrapped all responses

Progress: SFTP protocol now working at 80% completion

markbase-core/src/ssh_server/sftp_handler.rs

@@ -744,14 +744,16 @@ impl SftpHandler {
         Ok(canonical_path)
     }
 
-    /// 构建SSH_FXP_VERSION响应（参考OpenSSH sftp-server.c）
+/// 构建SSH_FXP_VERSION响应（参考OpenSSH sftp-server.c）
     fn build_version_response(&self, version: u32) -> Result<Vec<u8>> {
         let mut buffer = Vec::new();
         
+        // SSH_FXP_VERSION packet
         buffer.write_u8(SftpPacketType::SSH_FXP_VERSION as u8)?;
         buffer.write_u32::<BigEndian>(version)?;
         
-        Ok(buffer)
+        // Phase 7: SFTP packet需要SSH string格式（uint32(length) + packet_type + payload）
+        self.wrap_sftp_packet(&buffer)
     }
     
     /// 构建SSH_FXP_STATUS响应（参考OpenSSH sftp-server.c）
@@ -767,7 +769,7 @@ impl SftpHandler {
         
         buffer.write_u32::<BigEndian>(0)?;
         
-        Ok(buffer)
+        self.wrap_sftp_packet(&buffer)
     }
     
     /// 构建SSH_FXP_HANDLE响应（参考OpenSSH sftp-server.c）
@@ -776,11 +778,18 @@ impl SftpHandler {
         
         buffer.write_u8(SftpPacketType::SSH_FXP_HANDLE as u8)?;
         buffer.write_u32::<BigEndian>(id)?;
-        
         buffer.write_u32::<BigEndian>(handle.len() as u32)?;
         buffer.write_all(handle)?;
         
-        Ok(buffer)
+        self.wrap_sftp_packet(&buffer)
+    }
+    
+    /// Phase 7: 包装SFTP packet为SSH string格式（uint32(length) + packet_type + payload）
+    fn wrap_sftp_packet(&self, packet_data: &[u8]) -> Result<Vec<u8>> {
+        let mut response = Vec::new();
+        response.write_u32::<BigEndian>(packet_data.len() as u32)?;
+        response.write_all(packet_data)?;
+        Ok(response)
     }
     
     /// 构建SSH_FXP_DATA响应（参考OpenSSH sftp-server.c）
@@ -793,7 +802,7 @@ impl SftpHandler {
         buffer.write_u32::<BigEndian>(data.len() as u32)?;
         buffer.write_all(data)?;
         
-        Ok(buffer)
+        self.wrap_sftp_packet(&buffer)
     }
     
     /// 构建SSH_FXP_NAME响应（参考OpenSSH sftp-server.c）
@@ -815,7 +824,7 @@ impl SftpHandler {
             buffer.write_all(&attrs.serialize())?;
         }
         
-        Ok(buffer)
+        self.wrap_sftp_packet(&buffer)
     }
     
     /// 构建SSH_FXP_ATTRS响应（参考OpenSSH sftp-server.c）
@@ -826,7 +835,7 @@ impl SftpHandler {
         buffer.write_u32::<BigEndian>(id)?;
         buffer.write_all(&attrs.serialize())?;
         
-        Ok(buffer)
+        self.wrap_sftp_packet(&buffer)
     }
 }