fix: harden websocket heartbeat under load

loadtest/im-pressure/wsclient_test.go

@@ -84,3 +84,58 @@ func TestWSClientReadLoopClosesConnectionAfterDecodeError(t *testing.T) {
 		t.Fatal("read loop exited without closing the underlying connection")
 	}
 }
+
+func TestWSClientDataCallbackDoesNotBlockPong(t *testing.T) {
+	pong := make(chan struct{}, 1)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		conn, err := (&websocket.Upgrader{}).Upgrade(w, r, nil)
+		if err != nil {
+			return
+		}
+		defer conn.Close()
+		conn.SetPongHandler(func(string) error {
+			select {
+			case pong <- struct{}{}:
+			default:
+			}
+			return nil
+		})
+		if err = conn.WriteJSON(map[string]any{"data": map[string]any{
+			"topic": "grp-test", "seq": 1, "head": map[string]any{}, "content": map[string]any{},
+		}}); err != nil {
+			return
+		}
+		if err = conn.WriteControl(websocket.PingMessage, nil, time.Now().Add(time.Second)); err != nil {
+			return
+		}
+		conn.SetReadDeadline(time.Now().Add(time.Second))
+		for {
+			if _, _, readErr := conn.ReadMessage(); readErr != nil {
+				return
+			}
+		}
+	}))
+	defer server.Close()
+
+	conn, _, err := websocket.DefaultDialer.Dial("ws"+strings.TrimPrefix(server.URL, "http"), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	blocked := make(chan struct{})
+	client := &WSClient{
+		conn: conn, waits: make(map[string]chan ctrlPacket), done: make(chan struct{}),
+		onData: func(AccountRecord, dataPacket) { <-blocked },
+	}
+	client.startDataProcessor()
+	go client.readLoop()
+	defer func() {
+		close(blocked)
+		client.Close()
+	}()
+
+	select {
+	case <-pong:
+	case <-time.After(2 * time.Second):
+		t.Fatal("pong was blocked by the data callback")
+	}
+}