refactor: 统一类型注解风格并优化代码结构

- 将裸 except 改为显式 Exception 捕获
- 用列表推导式替换冗余 for 循环
- 为类属性添加 ClassVar 注解
- 统一 Union/Optional 写法为 |
- 移除未使用的导入
- 修复 SQLAlchemy 空值比较语法
- 优化字符串拼接与字典更新逻辑
- 补充缺失的 noqa 注释与异常链

BREAKING CHANGE: 所有插件基类的类级字段现要求显式 ClassVar 注解，自定义插件需同步更新

src/common/database/connection_pool_manager.py

@@ -190,11 +190,10 @@ class ConnectionPoolManager:
     async def _cleanup_expired_connections_locked(self):
         """清理过期连接（需要在锁内调用）"""
         time.time()
-        expired_connections = []
-
-        for connection_info in list(self._connections):
-            if connection_info.is_expired(self.max_lifetime, self.max_idle) and not connection_info.in_use:
-                expired_connections.append(connection_info)
+        expired_connections = [
+            connection_info for connection_info in list(self._connections)
+            if connection_info.is_expired(self.max_lifetime, self.max_idle) and not connection_info.in_use
+        ]
 
         for connection_info in expired_connections:
             await connection_info.close()