PHP 8.5: ArrayIterator deprecation in TwigExtension::withoutFilter()

Problem/Motivation

On PHP 8.5, TwigExtension::withoutFilter() can trigger: Deprecated function: ArrayIterator::__construct(): Using an object as a backing array for ArrayIterator is deprecated... in TwigExtension::withoutFilter() when flattening filter arguments via
RecursiveArrayIterator / RecursiveIteratorIterator.

This happens when a nested value in $args is an object — commonly when a theme mistakenly passes a render element instead of a string key, e.g.: {{ content|without(content.field_example) }} instead of: {{ content|without('field_example') }}

With error display enabled, the deprecation output can be emitted during rendering and contribute to "headers already sent" / session failures.

Related: #3543711: Fix PHP 8.5 deprecations of chr() and array iterator fixed other ArrayIterator PHP 8.5 cases, but not withoutFilter(). #3093577: Let Twig without() filter take both arrays and strings as arguments introduced RecursiveArrayIterator here.

Steps to reproduce

1. Run Drupal on PHP 8.5 with verbose error reporting.
2. Render a template that calls |without() with a render array / object nested in the arguments (or use a unit test that passes ['#object' => new \stdClass()] among the args).
3. Observe the ArrayIterator::__construct() deprecation.

Proposed resolution

Collect only scalar (and NULL) keys with array_walk_recursive() instead of RecursiveArrayIterator, so nested objects are ignored and no longer trigger the PHP 8.5 deprecation. Existing string/array-of-strings argument behavior is preserved.