Phase 2: Fix SSH_FXP_ATTRS uid/gid fields (resolve "? 0 0" issue)

- Phase 2.2: Add MetadataExt import to get uid/gid from file metadata
- Phase 2.3: Add SSH_FILEXFER_ATTR_UIDGID flag to attrs.flags
- Phase 2.4: Get uid/gid from metadata.uid() and metadata.gid()
- Result: ls -la now shows correct uid (501) and gid (0) instead of "? 0 0"

Root cause: SSH_FILEXFER_ATTR_UIDGID flag was missing, so uid/gid not serialized
Fix: Add flag and get uid/gid using std::os::unix::fs::MetadataExt

Test verification:
- Before: -rw-r--r--    ? 0        0            1024
- After:  -rw-r--r--    ? 501      0            1024  ✅

Reference: OpenSSH sftp-server.c: stat_to_attrib()

markbase-core/src/ssh_server/sftp_handler.rs

@@ -9,6 +9,7 @@ use std::path::{Path, PathBuf};
 use std::fs::{self, File, OpenOptions};
 use std::io::{Read, Write, Seek, SeekFrom};
 use std::os::unix::fs::PermissionsExt;  // 导入PermissionsExt trait（Unix标准）
+use std::os::unix::fs::MetadataExt;      // ⭐⭐⭐⭐⭐ Phase 2.2: 导入MetadataExt trait（获取uid/gid）
 
 /// SFTP packet类型（参考draft-ietf-secsh-filexfer-02.txt）
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -154,13 +155,19 @@ impl SftpAttrs {
     pub fn from_metadata(metadata: &fs::Metadata) -> Self {
         let mut attrs = Self::new();
         
+        // ⭐⭐⭐⭐⭐ Phase 2.2: 添加 uid/gid 字段（修复 "? 0 0" 权限显示问题）
         attrs.flags = SftpAttrFlags::SSH_FILEXFER_ATTR_SIZE
+            | SftpAttrFlags::SSH_FILEXFER_ATTR_UIDGID        // ⭐⭐⭐⭐⭐ 添加 UIDGID flag
             | SftpAttrFlags::SSH_FILEXFER_ATTR_PERMISSIONS
             | SftpAttrFlags::SSH_FILEXFER_ATTR_ACMODTIME;
         
         attrs.size = Some(metadata.len());
         attrs.permissions = Some(metadata.permissions().mode());
         
+        // ⭐⭐⭐⭐⭐ Phase 2.2: 获取 uid/gid（参考OpenSSH sftp-server.c: stat_to_attrib）
+        attrs.uid = Some(metadata.uid());
+        attrs.gid = Some(metadata.gid());
+        
         if let Ok(atime) = metadata.accessed() {
             attrs.atime = Some(atime.duration_since(std::time::UNIX_EPOCH).unwrap().as_secs() as u32);
         }