Annotate return value of validateOperationParams as list

The list built with `$errors[] = …` exclusively, which means it
cannot have gaps.

The helps libraries using this method, as they can type the
result passing through as `list<…>` as well.

Author: mfn

docs/class-reference.md

@@ -2225,7 +2225,7 @@ function parseRequestParams(string $method, array $bodyParams, array $queryParam
  * Checks validity of OperationParams extracted from HTTP request and returns an array of errors
  * if params are invalid (or empty array when params are valid).
  *
- * @return array<int, RequestError>
+ * @return list<RequestError>
  *
  * @api
  */

src/Server/Helper.php

@@ -140,7 +140,7 @@ public function parseRequestParams(string $method, array $bodyParams, array $que
      * Checks validity of OperationParams extracted from HTTP request and returns an array of errors
      * if params are invalid (or empty array when params are valid).
      *
-     * @return array<int, RequestError>
+     * @return list<RequestError>
      *
      * @api
      */