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/Migrations/ManagingDbContextModelSnapshot.cs

@@ -217,6 +217,9 @@ namespace Managing.Infrastructure.Databases.Migrations
                     b.Property<decimal>("NetPnl")
                         .HasColumnType("decimal(18,8)");
 
+                    b.Property<int>("PositionCount")
+                        .HasColumnType("integer");
+
                     b.Property<string>("PositionsJson")
                         .IsRequired()
                         .HasColumnType("jsonb");