add more controllers and fix tests

app/Services/PurchaseOrders/PurchaseOrderStatusService.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace App\Services\PurchaseOrders;
+
+use App\Models\PurchaseOrder;
+use RuntimeException;
+
+class PurchaseOrderStatusService
+{
+    public function cancel(int $poId): PurchaseOrder
+    {
+        $po = PurchaseOrder::query()->find($poId);
+        if (!$po || $po->status === PurchaseOrder::STATUS_RECEIVED) {
+            throw new RuntimeException('Cannot cancel this PO.');
+        }
+
+        $po->status = PurchaseOrder::STATUS_CANCELED;
+        $po->save();
+
+        return $po;
+    }
+}