Fix code quality: trailing whitespace, unused imports, clippy warnings

- Fix trailing whitespace in kex.rs and s3.rs
- Add missing KexProposal import in kex_complete.rs
- Auto-fix clippy warnings across all crates
- All 153 tests pass

markbase-core/src/vfs/mod.rs

@@ -1,5 +1,5 @@
-pub mod open_flags;
 pub mod local_fs;
+pub mod open_flags;
 pub mod s3_fs;
 pub mod util;
 
@@ -120,7 +120,11 @@ pub trait VfsBackend: Send {
     fn read_dir(&self, path: &Path) -> Result<Vec<VfsDirEntry>, VfsError>;
 
     /// 打开文件（读/写）
-    fn open_file(&self, path: &Path, flags: &open_flags::OpenFlags) -> Result<Box<dyn VfsFile>, VfsError>;
+    fn open_file(
+        &self,
+        path: &Path,
+        flags: &open_flags::OpenFlags,
+    ) -> Result<Box<dyn VfsFile>, VfsError>;
 
     /// 获取文件/目录元数据
     fn stat(&self, path: &Path) -> Result<VfsStat, VfsError>;