fix: 添加缺失的 callback 导入

- 在 config_flow.py 中添加 `from homeassistant.core import callback`
- 修复 "Invalid handler specified" 错误

custom_components/sigmesh_gateway/config_flow.py

@@ -6,6 +6,7 @@ from typing import Any
 
 import voluptuous as vol
 from homeassistant import config_entries
+from homeassistant.core import callback
 from homeassistant.data_entry_flow import FlowResult
 from homeassistant.helpers import selector