Phase 4: Implement SSH packet size limit (maxpack - 1024)

- Add maxpack field to SftpHandler structure
- Modify SftpHandler::new() to accept maxpack parameter
- Limit SSH_FXP_READ data size to maxpack - 1024 bytes (OpenSSH style)
- Get maxpack from Channel.remote_maxpacket

Changes:
- sftp_handler.rs: SftpHandler struct + new() + handle_read()
- channel.rs: Pass remote_maxpacket to SftpHandler::new()

Reference: OpenSSH sftp-server.c: process_read()
- Limit: maxpacket - 1024 bytes
- Prevent packet size violation

Test status: 5MB upload still incomplete (2.0MB)
- Issue may require additional debugging
- Upload direction may also need maxpack limit

markbase-core/src/ssh_server/channel.rs

@@ -460,7 +460,15 @@ impl ChannelManager {
             
             // Phase 7: 初始化SFTP handler
             let root_dir = PathBuf::from("/Users/accusys/markbase");  // 默认root目录
-            let sftp_handler = SftpHandler::new(root_dir);
+            
+            // ⭐⭐⭐⭐⭐ Phase 4: 获取 client maxpack 限制（从 Channel 中获取）
+            let maxpacket = if let Some(ch) = self.channels.get(&channel) {
+                ch.remote_maxpacket  // 来自 SSH_MSG_CHANNEL_OPEN 的 maximum_packet_size
+            } else {
+                32768  // OpenSSH 默认值（32KB）
+            };
+            
+            let sftp_handler = SftpHandler::new(root_dir, maxpacket);  // ⭐⭐⭐⭐⭐ Phase 4: 传入 maxpack
             
             // 存储到channel
             if let Some(ch) = self.channels.get_mut(&channel) {