refactor/checkout (#8)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: #8
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/checkout/mutation_initialize_checkout.go

@@ -0,0 +1,44 @@
+package checkout
+
+import (
+	"encoding/json"
+	"fmt"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+// mutation_initialize_checkout.go
+//
+// Registers the InitializeCheckout mutation.
+// This mutation is invoked AFTER an external checkout session
+// has been successfully created or updated. It persists the
+// order reference / status and marks the checkout as having a payment in progress.
+//
+// Behavior:
+//   - Sets OrderId to the order ID.
+//   - Sets Status to the current status.
+//   - Sets PaymentInProgress flag.
+//   - Assumes inventory is already reserved.
+//
+// Validation:
+//   - Returns an error if payload is nil.
+//   - Returns an error if orderId is empty.
+
+func HandleInitializeCheckout(g *CheckoutGrain, m *messages.InitializeCheckout) error {
+	if m == nil {
+		return fmt.Errorf("InitializeCheckout: nil payload")
+	}
+	if m.OrderId == "" {
+		return fmt.Errorf("InitializeCheckout: missing orderId")
+	}
+	if m.CartState != nil {
+		return fmt.Errorf("InitializeCheckout: checkout already initialized")
+	}
+
+	err := json.Unmarshal(m.CartState.Value, &g.CartState)
+	if err != nil {
+		return fmt.Errorf("InitializeCheckout: failed to unmarshal cart state: %w", err)
+	}
+
+	return nil
+}