fix: use explicit nullable types for PHP 8.4 compatibility (#309)

Replace all remaining implicit nullable parameter types with explicit
nullable type declarations to resolve PHP 8.4 deprecation warnings.

PR #306 previously fixed src/Billable.php; this commit addresses the
remaining 13 occurrences across exception constructors, interval
generators, Invoice methods, and test helpers.

Author: phillipfickl

src/Exceptions/AmountExceedsMolliePaymentMethodLimit.php

@@ -14,7 +14,7 @@ class AmountExceedsMolliePaymentMethodLimit extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The order total is greater than the total accepted by Mollie', int $code = 500, Throwable $previous = null)
+    public function __construct(string $message = 'The order total is greater than the total accepted by Mollie', int $code = 500, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/CouponException.php

@@ -14,7 +14,7 @@ class CouponException extends Exception
      * @param  int  $code
      * @param  \Throwable|null  $previous
      */
-    public function __construct(string $message, int $code = 404, Throwable $previous = null)
+    public function __construct(string $message, int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/CouponNotFoundException.php

@@ -13,7 +13,7 @@ class CouponNotFoundException extends CouponException
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Coupon not found', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Coupon not found', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/CurrencyMismatchException.php

@@ -14,7 +14,7 @@ class CurrencyMismatchException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Currencies do not match', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Currencies do not match', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/InvalidMandateException.php

@@ -14,7 +14,7 @@ class InvalidMandateException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Invalid customer mandate', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Invalid customer mandate', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/MandateIsNotYetFinalizedException.php

@@ -14,7 +14,7 @@ class MandateIsNotYetFinalizedException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The customer mandate is still pending', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'The customer mandate is still pending', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/OrderRetryRequiresStatusFailedException.php

@@ -14,7 +14,7 @@ class OrderRetryRequiresStatusFailedException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'The order status is not failed', int $code = 500, Throwable $previous = null)
+    public function __construct(string $message = 'The order status is not failed', int $code = 500, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Exceptions/PlanNotFoundException.php

@@ -14,7 +14,7 @@ class PlanNotFoundException extends Exception
      * @param  int  $code
      * @param  Throwable|null  $previous
      */
-    public function __construct(string $message = 'Plan not found', int $code = 404, Throwable $previous = null)
+    public function __construct(string $message = 'Plan not found', int $code = 404, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }

src/Order/Invoice.php

@@ -461,7 +461,7 @@ public function view(array $data = [], string $view = self::DEFAULT_VIEW)
      * @param  \Dompdf\Options  $options
      * @return string
      */
-    public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, Options $options = null)
+    public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, ?Options $options = null)
     {
         if (!defined('DOMPDF_ENABLE_AUTOLOAD')) {
             define('DOMPDF_ENABLE_AUTOLOAD', false);
@@ -482,7 +482,7 @@ public function pdf(array $data = [], string $view = self::DEFAULT_VIEW, Options
      * @param  \Dompdf\Options  $options
      * @return \Symfony\Component\HttpFoundation\Response
      */
-    public function download(array $data = [], string $view = self::DEFAULT_VIEW, Options $options = null)
+    public function download(array $data = [], string $view = self::DEFAULT_VIEW, ?Options $options = null)
     {
         $filename = implode('_', [
             $this->id,

src/Plan/AdvancedIntervalGenerator.php

@@ -26,7 +26,7 @@ public function __construct(array $configuration)
      * @param  \Laravel\Cashier\Subscription|null  $subscription
      * @return \Carbon\Carbon|\Carbon\Traits\Modifiers
      */
-    public function getEndOfNextSubscriptionCycle(Subscription $subscription = null)
+    public function getEndOfNextSubscriptionCycle(?Subscription $subscription = null)
     {
         $cycle_ends_at = $subscription->cycle_ends_at ?? now();
         $subscription_date = $this->startOfTheSubscription($subscription);