Add PositionCount property to Backtest models and responses

- Introduced PositionCount to Backtest, LightBacktest, and their respective response models.
- Updated BacktestController and BacktestExecutor to include PositionCount in responses.
- Modified database schema to accommodate new PositionCount field in relevant entities.

src/Managing.Api/Controllers/BacktestController.cs

@@ -316,7 +316,8 @@ public class BacktestController : BaseController
                 Score = b.Score,
                 ScoreMessage = b.ScoreMessage,
                 InitialBalance = b.InitialBalance,
-                NetPnl = b.NetPnl
+                NetPnl = b.NetPnl,
+                PositionCount = b.PositionCount
             }),
             TotalCount = totalCount,
             CurrentPage = page,
@@ -457,7 +458,8 @@ public class BacktestController : BaseController
                 Score = b.Score,
                 ScoreMessage = b.ScoreMessage,
                 InitialBalance = b.InitialBalance,
-                NetPnl = b.NetPnl
+                NetPnl = b.NetPnl,
+                PositionCount = b.PositionCount
             }),
             TotalCount = totalCount,
             CurrentPage = page,