Update Phase 6: Fix SFTP subsystem initialization and data handling

Phase 6 updates:
- Add SftpHandler integration in Channel structure
- Initialize SFTP handler on subsystem request
- Handle SFTP packets via CHANNEL_DATA
- Fix CHANNEL_DATA response handling in server loop

Phase 7 progress:
- SFTP subsystem initialization working
- SSH_FXP_INIT/VERSION handshake working
- SFTP packet format partially implemented
- Need further debugging for complete SFTP functionality

Current status:
- SSH command execution fully working (Phase 6 ✓)
- SFTP connection initialization working
- File transfer operations pending debug

markbase-core/src/ssh_server/server.rs

@@ -330,7 +330,12 @@ fn handle_ssh_service_loop(
             }
             Some(&pt) if pt == PacketType::SSH_MSG_CHANNEL_DATA as u8 => {
                 info!("Received SSH_MSG_CHANNEL_DATA");
-                channel_manager.handle_channel_data(&packet)?;
+                if let Some(response) = channel_manager.handle_channel_data(&packet)? {
+                    // Phase 7: SFTP响应通过CHANNEL_DATA返回
+                    let encrypted_response = EncryptedPacket::new(&response.payload, encryption_ctx, true)?;
+                    encrypted_response.write(stream)?;
+                    info!("Sent SSH_MSG_CHANNEL_DATA (SFTP response)");
+                }
             }
             Some(&pt) if pt == PacketType::SSH_MSG_CHANNEL_CLOSE as u8 => {
                 info!("Received SSH_MSG_CHANNEL_CLOSE");