fix: 修复 asyncio 任务清理和 StreamWriter.write() 协程错误

问题1: StreamWriter.write() 不是协程，不需要 await，错误使用会导致 TypeError
问题2: 隧道关闭时未正确取消转发任务，导致 'Task was destroyed but it is pending!' 警告
问题3: 未对 None 的 reader/writer 做前置检查，导致空指针异常

修复内容:
- 移除 dst.write(data) 前的 await
- _tunnel() finally 块中主动取消转发任务并 await
- _forward() 开头添加 None 检查
- _close_streams() 增加 None 检查
- 添加异常捕获日志

engine/server.py

@@ -328,6 +328,7 @@ class ConnectionHandler:
 
     async def _tunnel(self, dst_reader, dst_writer):
         """双向隧道转发。"""
+        f1 = f2 = None
         try:
             # 获取限速参数
             user = None
@@ -352,12 +353,22 @@ class ConnectionHandler:
         except Exception as e:
             log.exception("[%s] 隧道异常: %s", self.conn_id, e)
         finally:
+            # 确保转发任务被取消
+            for task in [f1, f2]:
+                if task and not task.done():
+                    task.cancel()
+                    try:
+                        await task
+                    except asyncio.CancelledError:
+                        pass
             self._close_streams(dst_reader, dst_writer)
             # 记录最终统计
             await self._record_stats()
 
     async def _forward(self, src, dst, direction, speed_limit_mbps, user):
         """单向转发，带限速。"""
+        if src is None or dst is None:
+            return
         total_transferred = 0
         try:
             while True:
@@ -372,7 +383,7 @@ class ConnectionHandler:
                     if sleep_time > 0.001:  # 小于 1ms 不节流
                         await asyncio.sleep(min(sleep_time, 1.0))
 
-                await dst.write(data)
+                dst.write(data)  # write() 不是协程，不需要 await
                 await dst.drain()
                 total_transferred += len(data)
 
@@ -385,10 +396,14 @@ class ConnectionHandler:
             pass
         except asyncio.CancelledError:
             pass
+        except Exception as e:
+            log.debug("[%s] 转发异常 %s: %s", self.conn_id, direction, e)
 
     def _close_streams(self, dst_reader, dst_writer):
         try:
+            if dst_writer is not None:
                 dst_writer.close()
+            if dst_reader is not None:
                 try:
                     dst_reader.feed_eof()
                 except Exception: