Verify Anikeen ID webhooks

Webhook::constructEvent() checks the Anikeen-Signature header (HMAC-SHA256
of "t.body" with the endpoint's secret, 5-minute tolerance) and returns
the event, like Stripe's helper.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Id/WebhookTest.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\Exceptions\SignatureVerificationException;
+use Anikeen\Id\Webhooks\Webhook;
+
+class WebhookTest extends TestCase
+{
+    private string $secret = 'whsec_test';
+
+    private string $payload = '{"id":"01JEVT","type":"order.completed","created":1790000000,"data":{"object":{"id":"01JORD"}}}';
+
+    private function header(?int $timestamp = null, ?string $secret = null): string
+    {
+        $timestamp ??= time();
+
+        return "t={$timestamp},v1=" . hash_hmac('sha256', "{$timestamp}.{$this->payload}", $secret ?? $this->secret);
+    }
+
+    public function test_a_signed_event_is_returned(): void
+    {
+        $event = Webhook::constructEvent($this->payload, $this->header(), $this->secret);
+
+        $this->assertSame('order.completed', $event['type']);
+        $this->assertSame('01JORD', $event['data']['object']['id']);
+    }
+
+    public function test_a_wrong_secret_is_rejected(): void
+    {
+        $this->expectException(SignatureVerificationException::class);
+
+        Webhook::constructEvent($this->payload, $this->header(secret: 'whsec_other'), $this->secret);
+    }
+
+    public function test_a_changed_body_is_rejected(): void
+    {
+        $this->expectException(SignatureVerificationException::class);
+
+        Webhook::constructEvent(str_replace('01JORD', '01JXXX', $this->payload), $this->header(), $this->secret);
+    }
+
+    public function test_old_requests_are_rejected(): void
+    {
+        $this->expectException(SignatureVerificationException::class);
+
+        Webhook::constructEvent($this->payload, $this->header(time() - 600), $this->secret);
+    }
+
+    public function test_a_missing_header_is_rejected(): void
+    {
+        $this->expectException(SignatureVerificationException::class);
+
+        Webhook::constructEvent($this->payload, null, $this->secret);
+    }
+}