feat(payments): add optional amount to VoidRequest for partial voids

lib/Checkout/Payments/VoidRequest.php

@@ -4,6 +4,13 @@
 
 class VoidRequest
 {
+    /**
+     * The amount to void. If not specified, the full payment amount is voided.
+     * Min 0, max 9999999999.
+     * @var int
+     */
+    public $amount;
+
     /**
      * @var string
      */

test/Checkout/Tests/Payments/VoidRequestSerializationTest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Checkout\Tests\Payments;
+
+use Checkout\JsonSerializer;
+use Checkout\Payments\VoidRequest;
+use PHPUnit\Framework\TestCase;
+
+class VoidRequestSerializationTest extends TestCase
+{
+    public function testVoidRequestSerializesAmountWhenSet()
+    {
+        $request = new VoidRequest();
+        $request->amount = 100;
+        $request->reference = "partial-void";
+
+        $decoded = json_decode((new JsonSerializer())->serialize($request), true);
+
+        $this->assertSame(100, $decoded['amount']);
+        $this->assertSame("partial-void", $decoded['reference']);
+    }
+
+    public function testVoidRequestOmitsAmountWhenUnset()
+    {
+        $request = new VoidRequest();
+        $request->reference = "full-void";
+
+        $decoded = json_decode((new JsonSerializer())->serialize($request), true);
+
+        $this->assertArrayNotHasKey('amount', $decoded);
+        $this->assertSame("full-void", $decoded['reference']);
+    }
+}