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/sftp_handler.rs

@@ -341,21 +341,6 @@ impl SftpHandler {
         let version = cursor.read_u32::<BigEndian>()?;
         info!("Client SFTP version: {}", version);
         
-        // Read any extension data client sent (SSH_FXP_INIT may contain extensions)
-        let pos = cursor.position() as usize;
-        let inner = cursor.get_ref();
-        if inner.len() > pos && (inner.len() - pos) >= 4 {
-            let ext_count = match cursor.read_u32::<BigEndian>() {
-                Ok(n) => n,
-                Err(_) => 0,
-            };
-            for i in 0..ext_count {
-                let ext_name = read_sftp_string(&mut cursor).unwrap_or_default();
-                let ext_data = read_sftp_string(&mut cursor).unwrap_or_default();
-                debug!("Client extension[{}]: {} = {}", i, ext_name, ext_data);
-            }
-        }
-        
         let response = self.build_version_response(3)?;
         Ok(response)
     }
@@ -376,8 +361,8 @@ impl SftpHandler {
         
         let full_path = self.resolve_path(&path)?;
         
-        let file = if pflags & SftpFileFlags::SSH_FXF_READ != 0 {
-            OpenOptions::new().read(true).open(&full_path).ok()
+        let file_result = if pflags & SftpFileFlags::SSH_FXF_READ != 0 {
+            OpenOptions::new().read(true).open(&full_path)
         } else if pflags & SftpFileFlags::SSH_FXF_WRITE != 0 {
             let mut opts = OpenOptions::new();
             opts.write(true);
@@ -393,13 +378,13 @@ impl SftpHandler {
             if pflags & SftpFileFlags::SSH_FXF_EXCL != 0 {
                 opts.create_new(true);
             }
-            opts.open(&full_path).ok()
+            opts.open(&full_path)
         } else {
-            None
+            return self.build_status_response(id, SftpStatus::SSH_FX_OP_UNSUPPORTED, "Unsupported open flags");
         };
         
-        match file {
-            Some(file) => {
+        match file_result {
+            Ok(file) => {
                 if self.handles.len() >= Self::MAX_HANDLES {
                     warn!("SSH_FXP_OPEN: handle limit reached ({})", Self::MAX_HANDLES);
                     return self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, "Handle limit reached");
@@ -419,8 +404,8 @@ impl SftpHandler {
                 
                 self.build_handle_response(id, &handle_id.to_be_bytes())
             }
-            None => {
-                self.build_status_response(id, SftpStatus::SSH_FX_FAILURE, "Failed to open file")
+            Err(e) => {
+                self.build_status_from_io_error(id, &e)
             }
         }
     }
@@ -1478,14 +1463,25 @@ impl SftpHandler {
         }
     }
 
-/// 构建SSH_FXP_VERSION响应，包含扩展声明（参考OpenSSH sftp-server.c）
+/// 构建SSH_FXP_VERSION响应，包含扩展声明（参考OpenSSH sftp-server.c: process_init()）
+    ///
+    /// SFTP协议格式（draft-ietf-secsh-filexfer-02）：
+    ///   uint32 length
+    ///   uint8 type (SSH_FXP_VERSION = 2)
+    ///   uint32 version
+    ///   // extensions: NO count field, simply paired strings until buffer empty
+    ///   string extension_name  (= uint32(len_with_nul) + data + \0)
+    ///   string extension_data  (= uint32(len_with_nul) + data + \0)
+    ///
+    /// OpenSSH uses sshbuf_put_cstring() which includes NUL terminator.
+    /// Client reads with sshbuf_get_cstring() which expects \0 at end.
     fn build_version_response(&self, version: u32) -> Result<Vec<u8>> {
         let mut buffer = Vec::new();
         
         buffer.write_u8(SftpPacketType::SSH_FXP_VERSION as u8)?;
         buffer.write_u32::<BigEndian>(version)?;
         
-        // 扩展声明（OpenSSH sftp-server.c: process_init() 中声明支持的扩展）
+        // 扩展声明 — OpenSSH sftp-server.c: process_init() style, NO count field
         let extensions: &[(&str, &str)] = &[
             ("posix-rename@openssh.com", "1"),
             ("hardlink@openssh.com", "1"),
@@ -1498,13 +1494,14 @@ impl SftpHandler {
             ("sha384-hash@openssh.com", "1"),
             ("sha512-hash@openssh.com", "1"),
         ];
-        
-        buffer.write_u32::<BigEndian>(extensions.len() as u32)?;
         for (name, data) in extensions {
-            buffer.write_u32::<BigEndian>(name.len() as u32)?;
+            // sshbuf_put_cstring(buf, s) → sshbuf_put_string(buf, s, strlen(s)+1)
+            buffer.write_u32::<BigEndian>((name.len() + 1) as u32)?;
             buffer.write_all(name.as_bytes())?;
-            buffer.write_u32::<BigEndian>(data.len() as u32)?;
+            buffer.write_u8(0)?;
+            buffer.write_u32::<BigEndian>((data.len() + 1) as u32)?;
             buffer.write_all(data.as_bytes())?;
+            buffer.write_u8(0)?;
         }
         
         self.wrap_sftp_packet(&buffer)