Add PHPUnit test suite

- Add PHPUnit 11.0 as dev dependency
- Add phpunit.xml configuration
- Add DTO tests (LicenseInfo, LicenseStatus, ActivationResult)
- Add Exception tests (factory method, all exception types)
- Add LicenseClient tests with mocked HTTP responses
- Update README with testing instructions
- Update CHANGELOG

32 tests, 93 assertions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/Dto/ActivationResultTest.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Magdev\WcLicensedProductClient\Tests\Dto;
+
+use Magdev\WcLicensedProductClient\Dto\ActivationResult;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(ActivationResult::class)]
+final class ActivationResultTest extends TestCase
+{
+    #[Test]
+    public function itCanBeCreatedFromArrayWithSuccess(): void
+    {
+        $data = [
+            'success' => true,
+            'message' => 'License activated successfully.',
+        ];
+
+        $result = ActivationResult::fromArray($data);
+
+        self::assertTrue($result->success);
+        self::assertSame('License activated successfully.', $result->message);
+    }
+
+    #[Test]
+    public function itCanBeCreatedFromArrayWithAlreadyActivated(): void
+    {
+        $data = [
+            'success' => true,
+            'message' => 'License is already activated for this domain.',
+        ];
+
+        $result = ActivationResult::fromArray($data);
+
+        self::assertTrue($result->success);
+        self::assertSame('License is already activated for this domain.', $result->message);
+    }
+}