Implement level 6 PHPStan fixes

tests/PjsonId.php

@@ -13,7 +13,7 @@ use Square\Pjson\JsonDataSerializable;
 /**
  * A serializable ID wrapper class
  */
-class PjsonId implements JsonDataSerializable
+final class PjsonId implements JsonDataSerializable
 {
     public function __construct(protected string $value) { }
 
@@ -22,6 +22,11 @@ class PjsonId implements JsonDataSerializable
         return $this->value;
     }
 
+    /**
+     * @param mixed $jd JSON data
+     * @param string[]|string $path The path within the data to be retrieved
+     * @return self
+     */
     public static function fromJsonData($jd, array|string $path = []): static
     {
         return new static($jd);