feat: add return types to few jsonSerialize()

phpseclib/Math/BigInteger.php

@@ -437,6 +437,8 @@ class BigInteger implements \JsonSerializable
      * JSON Serialize
      *
      * Will be called, automatically, when json_encode() is called on a BigInteger object.
+     *
+     * @return array{hex: string, precision?: int]
      */
     #[\ReturnTypeWillChange]
     public function jsonSerialize()

phpseclib/Math/BigInteger/Engines/Engine.php

@@ -370,6 +370,8 @@ abstract class Engine implements \JsonSerializable
      * JSON Serialize
      *
      * Will be called, automatically, when json_encode() is called on a BigInteger object.
+     *
+     * @return array{hex: string, precision?: int]
      */
     #[\ReturnTypeWillChange]
     public function jsonSerialize()

phpseclib/Math/Common/FiniteField/Integer.php

@@ -26,6 +26,8 @@ abstract class Integer implements \JsonSerializable
      *
      * PHP Serialize isn't supported because unserializing would require the factory be
      * serialized as well and that just sounds like too much
+     *
+     * @return array{hex: string}
      */
     #[\ReturnTypeWillChange]
     public function jsonSerialize()