macos_session: add with_writer for macOS

macos_session originally lacked the `with_writer` API.
This commit ports the implementation from `linux_session`
to provide a consistent interface across platforms.

Author: sidneychang

src/transport/fusedev/macos_session.rs

@@ -188,6 +188,19 @@ impl FuseSession {
             Err(SessionFailure("invalid fuse session".to_string()))
         }
     }
+    
+    /// Create a new fuse message channel with a specific buffer size.
+    pub fn with_writer<F>(&mut self, f: F)
+    where
+        F: FnOnce(FuseDevWriter),
+    {
+        if let Some(file) = &self.file {
+            let fd = file.as_raw_fd();
+            let mut buf = vec![0x0u8; self.bufsize];
+            let writer = FuseDevWriter::new(fd, &mut buf).unwrap();
+            f(writer);
+        }
+    }
 
     /// Wake channel loop
     /// After macfuse unmount, read will throw ENODEV