Use is_subclass_of pattern for consistency with existing codebase

Co-authored-by: lcharette <2566513+lcharette@users.noreply.github.com>

Author: Copilot

packages/sprinkle-core/app/src/Markdown/SprinkleMarkdownRepository.php

@@ -53,7 +53,7 @@ public function all(): array
                     throw new BadClassNameException("Extension class `$extensionClass` not found.");
                 }
                 $instance = $this->ci->get($extensionClass);
-                if (!$instance instanceof ExtensionInterface) {
+                if (!is_object($instance) || !is_subclass_of($instance, ExtensionInterface::class)) {
                     throw new BadInstanceOfException("Extension class `$extensionClass` doesn't implement " . ExtensionInterface::class . '.');
                 }
                 $instances[] = $instance;