fix: 修复 GeneratorExit 和 Task destroyed 警告

问题: 协程被取消/销毁时，finally 块中的异步操作触发 GeneratorExit
原因: GeneratorExit 是 BaseException 子类，不会被 Exception 捕获

修复内容:
- server.py handle(): 捕获 CancelledError + GeneratorExit
- server.py _tunnel(): 被取消时不执行 _record_stats() 异步操作
- instances.py _accept_connection(): 添加异常处理和 writer 关闭
- 所有取消场景都静默处理（不抛异常/不记录错误日志）

engine/instances.py

@@ -114,14 +114,23 @@ class Socks5Server:
         )
 
     async def _accept_connection(self, reader, writer):
-        with self._lock:
-            self._active_connections += 1
-        handler = ConnectionHandler(
-            reader, writer, self.config, self.user_service,
-            flask_app=self.flask_app,
-            on_close_cb=self._on_connection_close
-        )
-        await handler.handle()
+        try:
+            with self._lock:
+                self._active_connections += 1
+            handler = ConnectionHandler(
+                reader, writer, self.config, self.user_service,
+                flask_app=self.flask_app,
+                on_close_cb=self._on_connection_close
+            )
+            await handler.handle()
+        except (asyncio.CancelledError, GeneratorExit):
+            # 服务器关闭时正常取消，不记录错误
+            pass
+        finally:
+            try:
+                writer.close()
+            except Exception:
+                pass
 
     def _on_connection_close(self, handler):
         with self._lock: