Fix 5MB SFTP download hang: batch process SFTP packets + WINDOW_ADJUST chaining

Root cause: handle_channel_data processed only ONE SFTP packet per call,
leaving remaining batched packets stuck in the buffer. Client waited for
READ responses while server waited for more data — deadlock after ~3.1MB.

Fix:
- sftp_handler.rs: fix SSH_FXP_VERSION format (remove uint32 extension_count)
- sftp_handler.rs: fix handle_open error mapping (.ok() → build_status_from_io_error)
- channel.rs: batch-process ALL complete SFTP packets from buffer in loop
- channel.rs: add pending_packets VecDeque for multi-response queuing
- channel.rs: chain WINDOW_ADJUST + SFTP response when window is low
- channel.rs: add adjust_remote_window() for client WINDOW_ADJUST
- server.rs: drain pending_packets after each CHANNEL_DATA handler

Verified: 5MB upload + download with matching MD5

markbase-core/src/ssh_server/server.rs

@@ -501,6 +501,13 @@ fn handle_ssh_service_loop(
                     encrypted_response.write(stream)?;
                     info!("Sent SSH_MSG_CHANNEL_DATA (SFTP response)");
                 }
+                
+                // ⭐⭐⭐⭐⭐ Phase 15.1: Drain pending packets (e.g. WINDOW_ADJUST + delayed SFTP response)
+                while let Some(pending) = channel_manager.pending_packets.pop_front() {
+                    let encrypted_pending = EncryptedPacket::new(&pending.payload, encryption_ctx, true)?;
+                    encrypted_pending.write(stream)?;
+                    info!("Sent pending packet (type {})", pending.payload.first().unwrap_or(&0));
+                }
             }
             Some(&pt) if pt == PacketType::SSH_MSG_CHANNEL_CLOSE as u8 => {
                 info!("Received SSH_MSG_CHANNEL_CLOSE");
@@ -518,6 +525,15 @@ fn handle_ssh_service_loop(
                 info!("Received SSH_MSG_DISCONNECT");
                 break;
             }
+            Some(&pt) if pt == PacketType::SSH_MSG_CHANNEL_WINDOW_ADJUST as u8 => {
+                let payload = &packet.payload;
+                if payload.len() >= 9 {
+                    // Format: uint32 recipient_channel || uint32 bytes_to_add
+                    let recipient_channel = u32::from_be_bytes([payload[1], payload[2], payload[3], payload[4]]);
+                    let bytes_to_add = u32::from_be_bytes([payload[5], payload[6], payload[7], payload[8]]);
+                    channel_manager.adjust_remote_window(recipient_channel, bytes_to_add);
+                }
+            }
             _ => {
                 warn!("Unknown packet type: {:?}", packet.payload.first());
             }