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.Infrastructure.Database/PostgreSql/Entities/BacktestEntity.cs

@@ -128,4 +128,7 @@ public class BacktestEntity
     [Required]
     [Column(TypeName = "decimal(18,8)")]
     public decimal NetPnl { get; set; }
+
+    [Required]
+    public int PositionCount { get; set; }
 }