Standardize nullable type hints to 'null|Type' order

Refactors type hints and docblocks across the codebase to consistently use the 'null|Type' order for nullable types, improving code readability and alignment with modern PHP standards. No functional changes are introduced; this is a documentation and type annotation update only.

Author: huangdijia

src/Annotation/ValidationAspect.php

@@ -61,7 +61,7 @@ public function process(ProceedingJoinPoint $proceedingJoinPoint)
     protected function getValidationRules(ProceedingJoinPoint $proceedingJoinPoint): array
     {
         $metadata = $proceedingJoinPoint->getAnnotationMetadata();
-        /** @var Validation|null $annotation */
+        /** @var null|Validation $annotation */
         $annotation = $metadata->method[Validation::class] ?? null;
 
         if (! $annotation) {