feat(F-087): completed feature

project/src/modules/cart/domain/errors.ts

@@ -4,3 +4,16 @@ export class InvalidCartQuantityError extends Error {
     this.name = 'InvalidCartQuantityError';
   }
 }
+
+export class InsufficientCartStockError extends Error {
+  constructor(
+    public readonly variantId: string,
+    public readonly requested: number,
+    public readonly available: number,
+  ) {
+    super(
+      `Insufficient stock for variant ${variantId}: requested ${requested}, available ${available}`,
+    );
+    this.name = 'InsufficientCartStockError';
+  }
+}