feat: AsyncTdxClient 增加心跳保活机制

1. 在 AsyncTdxClient 中增加后台心跳任务，默认每 60 秒发送一次请求。
2. 支持在连接成功后自动启动，并在连接关闭时自动停止心跳任务。
3. 增加单元测试 tests/unit/test_heartbeat.py 验证心跳循环与清理逻辑。

tests/unit/test_heartbeat.py

@@ -0,0 +1,65 @@
+"""心跳机制单元测试。"""
+
+import asyncio
+import pytest
+from unittest.mock import patch, MagicMock, AsyncMock
+from xmtdx import AsyncTdxClient, Market
+
+
+@pytest.mark.asyncio
+async def test_heartbeat_sends_periodically():
+    # 模拟连接和执行
+    with patch("xmtdx.client.AsyncTdxConnection") as mock_conn_cls:
+        mock_conn = mock_conn_cls.return_value
+        mock_conn.connect = AsyncMock()
+        mock_conn.close = AsyncMock()
+        
+        # 记录调用次数
+        call_count = 0
+        async def mock_execute(cmd):
+            nonlocal call_count
+            call_count += 1
+            return 5 # 模拟 get_security_count 返回值
+
+        mock_conn.execute.side_effect = mock_execute
+
+        # 设置非常短的心跳间隔以便测试
+        client = AsyncTdxClient("127.0.0.1", heartbeat_interval=0.1)
+        await client.connect()
+        
+        # 等待几次心跳周期
+        await asyncio.sleep(0.35)
+        
+        await client.close()
+        
+        # 0.35s 应该触发约 3 次心跳 (0.1, 0.2, 0.3)
+        assert call_count >= 3
+
+
+@pytest.mark.asyncio
+async def test_heartbeat_stops_on_close():
+    with patch("xmtdx.client.AsyncTdxConnection") as mock_conn_cls:
+        mock_conn = mock_conn_cls.return_value
+        mock_conn.connect = AsyncMock()
+        mock_conn.close = AsyncMock()
+        mock_conn.execute = AsyncMock(return_value=5)
+        
+        client = AsyncTdxClient("127.0.0.1", heartbeat_interval=0.01)
+        await client.connect()
+        assert client._heartbeat_task is not None
+        
+        task = client._heartbeat_task
+        await client.close()
+        
+        assert client._heartbeat_task is None
+        assert task.done() or task.cancelled()
+
+
+if __name__ == "__main__":
+    # 手动跑一下
+    async def run():
+        await test_heartbeat_sends_periodically()
+        await test_heartbeat_stops_on_close()
+        print("Heartbeat tests passed!")
+    
+    asyncio.run(run())