Add update-check endpoint support (v0.2.1)

Implement /update-check endpoint aligned with remote OpenAPI spec:
- Add checkForUpdates() method to LicenseClientInterface
- Add UpdateInfo DTO for update check responses
- Add ProductNotFoundException for product_not_found error
- Update local openapi.json to v0.4.0

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

src/LicenseClient.php

@@ -7,6 +7,7 @@ namespace Magdev\WcLicensedProductClient;
 use Magdev\WcLicensedProductClient\Dto\ActivationResult;
 use Magdev\WcLicensedProductClient\Dto\LicenseInfo;
 use Magdev\WcLicensedProductClient\Dto\LicenseStatus;
+use Magdev\WcLicensedProductClient\Dto\UpdateInfo;
 use Magdev\WcLicensedProductClient\Exception\LicenseException;
 use Psr\Cache\CacheItemPoolInterface;
 use Psr\Log\LoggerInterface;
@@ -144,6 +145,57 @@ final class LicenseClient implements LicenseClientInterface
         return $result;
     }
 
+    public function checkForUpdates(
+        string $licenseKey,
+        string $domain,
+        ?string $pluginSlug = null,
+        ?string $currentVersion = null,
+    ): UpdateInfo {
+        $cacheKey = $this->buildCacheKey('update-check', $licenseKey, $domain);
+
+        if ($this->cache !== null) {
+            $item = $this->cache->getItem($cacheKey);
+            if ($item->isHit()) {
+                $this->logger->debug('Update check cache hit', ['domain' => $domain]);
+                return $item->get();
+            }
+        }
+
+        $this->logger->info('Checking for updates', ['domain' => $domain]);
+
+        $payload = [
+            'license_key' => $licenseKey,
+            'domain' => $domain,
+        ];
+
+        if ($pluginSlug !== null) {
+            $payload['plugin_slug'] = $pluginSlug;
+        }
+
+        if ($currentVersion !== null) {
+            $payload['current_version'] = $currentVersion;
+        }
+
+        $response = $this->request('update-check', $payload);
+
+        $result = UpdateInfo::fromArray($response);
+
+        if ($this->cache !== null) {
+            $item = $this->cache->getItem($cacheKey);
+            $item->set($result);
+            $item->expiresAfter($this->cacheTtl);
+            $this->cache->save($item);
+        }
+
+        $this->logger->info('Update check completed', [
+            'domain' => $domain,
+            'update_available' => $result->updateAvailable,
+            'version' => $result->version,
+        ]);
+
+        return $result;
+    }
+
     /**
      * @throws LicenseException
      */