Update cost calculation logic in Position class

- Changed the condition in _update_costs method to check for trade.position_effect being POSITION_EFFECT.OPEN instead of trade.side being SIDE.BUY, ensuring accurate cost updates for trade transactions.
- This adjustment improves the handling of trade costs based on the position effect, enhancing the overall functionality of the Position class.

Author: Cuizi7

rqalpha/portfolio/position.py

@@ -248,7 +248,7 @@ def before_trading(self, trading_date):
 
     def _update_costs(self, trade: Trade):
         self._transaction_cost += trade.transaction_cost
-        if trade.side == SIDE.BUY:
+        if trade.position_effect == POSITION_EFFECT.OPEN:
             self._trade_cost += trade.last_price * trade.last_quantity
         else:
             self._trade_cost -= trade.last_price * trade.last_quantity