Add backend projects with core models and tests, setup Godot frontend

backend/tests/MyBiz.Tests/ProductTests.cs

@@ -0,0 +1,41 @@
+using MyBiz.Core;
+
+namespace MyBiz.Tests;
+
+public class ProductTests
+{
+    [Fact]
+    public void Product_Creation_ShouldInitializeProperties()
+    {
+        // Arrange & Act
+        var product = new Product
+        {
+            Type = ProductType.Food,
+            Name = "Bread",
+            Category = ProductCategory.ConsumerGoods,
+            BasePrice = 10m
+        };
+
+        // Assert
+        Assert.Equal(ProductType.Food, product.Type);
+        Assert.Equal("Bread", product.Name);
+        Assert.Equal(ProductCategory.ConsumerGoods, product.Category);
+        Assert.Equal(10m, product.BasePrice);
+    }
+    
+    [Fact]
+    public void Product_CurrentPrice_ShouldDefaultToBasePrice()
+    {
+        // Arrange
+        var product = new Product
+        {
+            BasePrice = 25m
+        };
+
+        // Act
+        product.CurrentPrice = product.BasePrice;
+
+        // Assert
+        Assert.Equal(25m, product.CurrentPrice);
+    }
+}