Fix WebSocket transport None race condition in proxy

Add a transport validity check before WebSocket upgrade to prevent
AssertionError when clients disconnect during handshake.

The issue occurs when a client connection is lost between the API state
check and server.prepare() call, causing request.transport to become None
and triggering "assert transport is not None" in aiohttp's _pre_start().

The fix detects the closed connection early and raises HTTPBadRequest
with a clear reason instead of crashing with an AssertionError.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/api/test_proxy.py

@@ -223,6 +223,34 @@ async def test_proxy_auth_abort_log(
         )
 
 
+async def test_websocket_transport_none(
+    coresys,
+    caplog: pytest.LogCaptureFixture,
+):
+    """Test WebSocket connection with transport None is handled gracefully."""
+    from aiohttp import web
+
+    # Get the API proxy instance from coresys
+    api_proxy = APIProxy.__new__(APIProxy)
+    api_proxy.coresys = coresys
+
+    # Create a mock request with transport set to None to simulate connection loss
+    mock_request = AsyncMock(spec=web.Request)
+    mock_request.transport = None
+
+    caplog.clear()
+    with caplog.at_level(logging.WARNING):
+        # This should raise HTTPBadRequest, not AssertionError
+        with pytest.raises(web.HTTPBadRequest) as exc_info:
+            await api_proxy.websocket(mock_request)
+
+        # Verify the error reason
+        assert exc_info.value.reason == "Connection closed"
+
+        # Verify the warning was logged
+        assert "WebSocket connection lost before upgrade" in caplog.text
+
+
 @pytest.mark.parametrize("path", ["", "mock_path"])
 async def test_api_proxy_get_request(
     api_client: TestClient,