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>

README.md

@@ -23,6 +23,7 @@ composer require magdev/wc-licensed-product-client
 - License validation against domains
 - License activation on domains
 - License status checking
+- Plugin update checking
 - Comprehensive exception handling
 - Code integrity verification
 - Built on Symfony HttpClient
@@ -52,6 +53,12 @@ echo "Status: " . $status->status->value;
 // Activate a license
 $result = $client->activate('ABCD-1234-EFGH-5678', 'example.com');
 echo "Activated: " . ($result->success ? 'Yes' : 'No');
+
+// Check for updates
+$updateInfo = $client->checkForUpdates('ABCD-1234-EFGH-5678', 'example.com', 'my-plugin', '1.0.0');
+if ($updateInfo->updateAvailable) {
+    echo "New version available: " . $updateInfo->version;
+}
 ```
 
 ### With Logging
@@ -160,6 +167,7 @@ This client interacts with the following WooCommerce Licensed Product API endpoi
 - **POST /validate** - Validate a license key for a specific domain
 - **POST /status** - Get detailed license status information
 - **POST /activate** - Activate a license on a domain
+- **POST /update-check** - Check for plugin updates
 
 ## License