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>

docs/client-implementation.md

@@ -40,6 +40,7 @@ composer require magdev/wc-licensed-product-client
 │ + validate()                │
 │ + status()                  │
 │ + activate()                │
+│ + checkForUpdates()         │
 └─────────────┬───────────────┘
               │ implements
               ├───────────────────────┐
@@ -57,6 +58,7 @@ composer require magdev/wc-licensed-product-client
               │ LicenseInfo         │
               │ LicenseStatus       │
               │ ActivationResult    │
+              │ UpdateInfo          │
               │ LicenseState (enum) │
               └─────────────────────┘
 ```
@@ -71,7 +73,8 @@ src/
 ├── Dto/
 │   ├── LicenseInfo.php           # Validation response DTO
 │   ├── LicenseStatus.php         # Status response DTO + LicenseState enum
-│   └── ActivationResult.php      # Activation response DTO
+│   ├── ActivationResult.php      # Activation response DTO
+│   └── UpdateInfo.php            # Update check response DTO
 ├── Exception/
 │   ├── LicenseException.php      # Base exception
 │   ├── LicenseNotFoundException.php
@@ -82,6 +85,7 @@ src/
 │   ├── DomainMismatchException.php
 │   ├── MaxActivationsReachedException.php
 │   ├── ActivationFailedException.php
+│   ├── ProductNotFoundException.php
 │   └── RateLimitExceededException.php
 └── Security/
     ├── ResponseSignature.php     # HMAC signature verification
@@ -120,6 +124,18 @@ interface LicenseClientInterface
      * @throws LicenseException When activation fails
      */
     public function activate(string $licenseKey, string $domain): ActivationResult;
+
+    /**
+     * Check for available plugin updates.
+     *
+     * @throws LicenseException When update check fails
+     */
+    public function checkForUpdates(
+        string $licenseKey,
+        string $domain,
+        ?string $pluginSlug = null,
+        ?string $currentVersion = null,
+    ): UpdateInfo;
 }
 ```
 
@@ -306,6 +322,57 @@ if ($result->success) {
 }
 ```
 
+### UpdateInfo
+
+Returned by `checkForUpdates()`. Contains plugin update information.
+
+```php
+final readonly class UpdateInfo
+{
+    public function __construct(
+        public bool $updateAvailable,           // Whether an update is available
+        public ?string $version,                // Latest available version
+        public ?string $slug,                   // Plugin slug for WordPress
+        public ?string $plugin,                 // Plugin basename (slug/slug.php)
+        public ?string $downloadUrl,            // Secure download URL
+        public ?\DateTimeImmutable $lastUpdated,// Date of the latest release
+        public ?string $tested,                 // Highest WordPress version tested
+        public ?string $requires,               // Minimum WordPress version
+        public ?string $requiresPhp,            // Minimum PHP version
+        public ?string $changelog,              // Release notes
+        public ?string $packageHash,            // SHA256 hash for integrity
+        public ?string $name,                   // Product name
+        public ?string $homepage,               // Product homepage URL
+        public ?array $icons,                   // Plugin icons for WordPress admin
+        public ?array $sections,                // Content sections for plugin info
+    ) {}
+
+    public static function fromArray(array $data): self;
+
+    public function hasValidPackageHash(): bool;  // Check package hash format
+}
+```
+
+**Usage:**
+
+```php
+$updateInfo = $client->checkForUpdates(
+    'ABCD-1234-EFGH-5678',
+    'example.com',
+    'my-plugin',
+    '1.0.0',
+);
+
+if ($updateInfo->updateAvailable) {
+    echo "New version available: " . $updateInfo->version;
+    echo "Download URL: " . $updateInfo->downloadUrl;
+
+    if ($updateInfo->hasValidPackageHash()) {
+        echo "Package hash: " . $updateInfo->packageHash;
+    }
+}
+```
+
 ## Exception Hierarchy
 
 All exceptions extend `LicenseException` and include an `errorCode` property for programmatic handling.
@@ -320,8 +387,9 @@ LicenseException (base)
 ├── DomainMismatchException       // error: domain_mismatch
 ├── MaxActivationsReachedException // error: max_activations_reached
 ├── ActivationFailedException     // error: activation_failed
+├── ProductNotFoundException      // error: product_not_found
 ├── RateLimitExceededException    // error: rate_limit_exceeded (has retryAfter)
-└── Security\SignatureException   // error: signature_invalid
+├── Security\SignatureException   // error: signature_invalid
 └── Security\IntegrityException   // error: integrity_check_failed
 ```
 
@@ -925,6 +993,7 @@ The client communicates with these REST API endpoints:
 | `/wp-json/wc-licensed-product/v1/validate` | POST | Validate license for domain |
 | `/wp-json/wc-licensed-product/v1/status` | POST | Get license status details |
 | `/wp-json/wc-licensed-product/v1/activate` | POST | Activate license on domain |
+| `/wp-json/wc-licensed-product/v1/update-check` | POST | Check for plugin updates |
 
 ### Request Format
 
@@ -974,6 +1043,7 @@ All requests use JSON bodies:
 | `domain_mismatch` | `DomainMismatchException` | Domain not authorized |
 | `max_activations_reached` | `MaxActivationsReachedException` | Too many activations |
 | `activation_failed` | `ActivationFailedException` | Server error during activation |
+| `product_not_found` | `ProductNotFoundException` | Licensed product doesn't exist |
 | `rate_limit_exceeded` | `RateLimitExceededException` | Too many requests |
 | `signature_invalid` | `SignatureException` | Response signature invalid |
 | `integrity_check_failed` | `IntegrityException` | Source files modified |