feat: add PHPStan level 5 with zero errors

- Install phpstan/phpstan, add phpstan.neon at level 5 targeting PHP 8.3
- Add phpstan-bootstrap.php stubs for Laravel helper functions (app,
  config, config_path, app_path, session) unavailable in standalone packages
- Fix CartCondition::setOrder() - removed incorrect @return int (method is void)
- Fix CartCondition::getCalculatedValue() - removed incorrect @return mixed PHPDoc
- Fix Cart::condition() - remove redundant instanceof check the type system already guarantees
- Fix Cart::fireEvent() - remove array_values() on already-list literal array
- Fix ItemCollection - add @property annotations for magic __get properties
- Fix ItemCollection::getAssociatedModel() - correct @return bool to @return mixed
- Fix ItemCollection::getConditions() - remove dead is_array && !instanceof check
- Fix Validator - add @method stub so PHPStan resolves make() through __callStatic
- Add ignoreErrors for trait.unused (HasCart designed for external Livewire use)
- Add ignoreErrors for round() mode mismatch (PHPStan 2.x PHP 8.4 stubs bug)
- Add phpstan step to CI (PHP 8.3 only, matches phpVersion config)

src/ItemCollection.php

@@ -5,6 +5,14 @@
 use Illuminate\Support\Collection;
 use Wearepixel\Cart\Helpers\Helpers;
 
+/**
+ * @property mixed $id
+ * @property string $name
+ * @property float|int $price
+ * @property int|float $quantity
+ * @property array<string, mixed> $attributes
+ * @property mixed $conditions
+ */
 class ItemCollection extends Collection
 {
     /**
@@ -46,7 +54,7 @@ public function __get($name)
     /**
      * return the associated model of an item
      *
-     * @return bool
+     * @return mixed
      */
     protected function getAssociatedModel()
     {
@@ -97,7 +105,7 @@ public function getConditions()
 
         // check if we're already an array of CartConditions
         foreach ($conditions as $key => $condition) {
-            if ($condition instanceof CartCondition) {
+            if (is_object($condition) && $condition instanceof CartCondition) {
                 return $conditions;
             }
         }
@@ -105,7 +113,7 @@ public function getConditions()
         $hasSubArray = false;
 
         foreach ($conditions as $key => $condition) {
-            if (is_array($condition) && ! $condition instanceof CartCondition) {
+            if (is_array($condition)) {
                 $hasSubArray = true;
                 $conditionsArray[] = new CartCondition($condition);
             } elseif ($condition instanceof CartCondition) {