Twig Sandbox: multiple `__toString()` policy bypasses via unguarded string coercion points

Twig is a template language for PHP. Prior to 3.26.0, several Twig language constructs trigger PHP string coercion on a Stringable operand without consulting SecurityPolicy::checkMethodAllowed(), allowing a sandboxed template author to invoke __toString() on objects reachable in the render context through conditional expressions, comparison operators, tests, template-loading tags, dynamic attribute names, spread arguments, the do tag, and the .. range operator. This issue is fixed in version 3.26.0.