feat: make event state constructor-only and immutable

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

tests/php/Unit/Event/EventTest.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace KTXT\Unit\Event;
+
+use KTXF\Event\Event;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\TestCase;
+
+final class EventTest extends TestCase
+{
+    #[Test]
+    #[TestDox('Event data and context are supplied during construction')]
+    public function constructsCompleteEventState(): void
+    {
+        $event = new Event(
+            'test.event',
+            ['nested' => ['value' => 'original']],
+            'tenant-a',
+            'identity-a',
+        );
+
+        $copy = $event->getData();
+        $copy['nested']['value'] = 'changed';
+
+        self::assertSame('original', $event->get('nested')['value']);
+        self::assertSame('tenant-a', $event->getTenantId());
+        self::assertSame('identity-a', $event->getIdentityId());
+    }
+
+    #[Test]
+    #[TestDox('Event data rejects mutable object references')]
+    public function rejectsMutablePayloadValues(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+
+        new Event('test.event', ['mutable' => new \stdClass()]);
+    }
+}