add basic usecase layer

src/use_cases/cart_repository_interface.py

@@ -0,0 +1,8 @@
+from typing import Optional, List
+
+class CartRepositoryInterface:
+    def fetch_cart_by_user_id(self, user_id: int) -> Optional[List[dict]]:
+        """
+        Abstract method to fetch cart data by user ID.
+        """
+        raise NotImplementedError("fetch_cart_by_user_id must be implemented by a subclass")

src/use_cases/view_cart.py

@@ -0,0 +1,22 @@
+#python imports
+from typing import Optional
+
+#dependency imports
+from entities.cart import Cart, CartItem
+from interface_adapters.cart_repository import CartRepository
+
+class ViewCart:
+    def __init__(self, cart_repository: CartRepository):
+        self.cart_repository = cart_repository
+
+    def execute(self, user_id: int) -> Optional[Cart]:
+        """
+        Fetches the cart data from the repository, converts it to Cart entity.
+        """
+        cart_data = self.cart_repository.fetch_cart_by_user_id(user_id)
+        if not cart_data:
+            return None
+
+        # Convert raw data to domain entities
+        items = [CartItem(**item) for item in cart_data]
+        return Cart(items)