A: Code quality improvements - fix clippy warnings

- Remove unused imports in server.rs (Body, HeaderValue, RwLock)
- Remove unused imports in forward_acl.rs (tests still need Ipv4Addr)
- Remove unused imports in host_key.rs (Read, Write)
- Remove unused imports in kex_exchange.rs (HostKeyType)
- Remove unused imports in known_hosts.rs (tests need Ipv4Addr)
- Remove unused imports in multiplex.rs (Arc)
- Auto-fix other unused imports via clippy --fix

Tests: 303 passed, 0 failed (4 new tests added)

markbase-core/src/vfs/async_fs.rs

@@ -57,6 +57,12 @@ pub struct AsyncLocalFs {
     root: PathBuf,
 }
 
+impl Default for AsyncLocalFs {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl AsyncLocalFs {
     pub fn new() -> Self {
         Self { root: PathBuf::new() }
@@ -153,7 +159,7 @@ impl super::AsyncVfsBackend for AsyncLocalFs {
         })
     }
 
-    fn create_dir<'a>(&'a self, path: &'a Path, mode: u32) -> Pin<Box<dyn Future<Output = Result<(), VfsError>> + Send + 'a>> {
+    fn create_dir<'a>(&'a self, path: &'a Path, _mode: u32) -> Pin<Box<dyn Future<Output = Result<(), VfsError>> + Send + 'a>> {
         Box::pin(async move {
             fs::create_dir(path).await
                 .map_err(|e| Self::map_io_error(path, e))?;