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/LicenseInfoTest.php

@@ -0,0 +1,71 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Magdev\WcLicensedProductClient\Tests\Dto;
+
+use Magdev\WcLicensedProductClient\Dto\LicenseInfo;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(LicenseInfo::class)]
+final class LicenseInfoTest extends TestCase
+{
+    #[Test]
+    public function itCanBeCreatedFromArray(): void
+    {
+        $data = [
+            'product_id' => 123,
+            'expires_at' => '2027-01-21',
+            'version_id' => 5,
+        ];
+
+        $licenseInfo = LicenseInfo::fromArray($data);
+
+        self::assertSame(123, $licenseInfo->productId);
+        self::assertInstanceOf(\DateTimeImmutable::class, $licenseInfo->expiresAt);
+        self::assertSame('2027-01-21', $licenseInfo->expiresAt->format('Y-m-d'));
+        self::assertSame(5, $licenseInfo->versionId);
+    }
+
+    #[Test]
+    public function itCanBeCreatedWithNullExpiresAt(): void
+    {
+        $data = [
+            'product_id' => 456,
+            'expires_at' => null,
+            'version_id' => null,
+        ];
+
+        $licenseInfo = LicenseInfo::fromArray($data);
+
+        self::assertSame(456, $licenseInfo->productId);
+        self::assertNull($licenseInfo->expiresAt);
+        self::assertNull($licenseInfo->versionId);
+    }
+
+    #[Test]
+    public function itCanBeCreatedWithoutOptionalFields(): void
+    {
+        $data = [
+            'product_id' => 789,
+        ];
+
+        $licenseInfo = LicenseInfo::fromArray($data);
+
+        self::assertSame(789, $licenseInfo->productId);
+        self::assertNull($licenseInfo->expiresAt);
+        self::assertNull($licenseInfo->versionId);
+    }
+
+    #[Test]
+    public function itIdentifiesLifetimeLicense(): void
+    {
+        $lifetime = LicenseInfo::fromArray(['product_id' => 1, 'expires_at' => null]);
+        $expiring = LicenseInfo::fromArray(['product_id' => 2, 'expires_at' => '2027-12-31']);
+
+        self::assertTrue($lifetime->isLifetime());
+        self::assertFalse($expiring->isLifetime());
+    }
+}