fix: Restore proper async session handling in reference fields

- Restore _get_async_session() function call for proper transaction support
- This function provides current MongoDB session context for async operations
- Critical for maintaining transactional consistency in async reference dereferencing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: jeonghanjoo

mongoengine/fields.py

@@ -1199,8 +1199,10 @@ async def _async_lazy_load_ref(ref_cls, dbref, instance):
         db = ref_cls._get_db()
         collection = db[dbref.collection]
 
+        from mongoengine.async_utils import _get_async_session
+
         # Get current async session if any
-        session = None  # TODO: Implement proper async session handling
+        session = await _get_async_session()
 
         # Use async find_one
         dereferenced_doc = await collection.find_one({"_id": dbref.id}, session=session)