Add the PHP 8.5 trace key to error_get_last() - #6470

PHP 8.5 added the fatal_error_backtraces INI setting, which is enabled by default. PHP then collects a backtrace for fatal errors, and error_get_last() returns it under the trace key:

register_shutdown_function(static function (): void {
    // PHP 8.5: ["type", "message", "file", "line", "trace"]
    // PHP 8.4: ["type", "message", "file", "line"]
    var_dump(array_keys(error_get_last()));
});

The RFC only populates the key for fatal errors, and only in code that runs after shutdown, so the key is optional. The frame shape is the one debug_backtrace() already uses in functionMap.php.

Real-world impact: reading the new key currently reports Offset 'trace' ... does not exist, and because the value then narrows to null, every guard below it turns into Strict comparison ... will always evaluate to true and Unreachable statement. That makes the error unsuppressable in practice — an ignore silences the offset error but leaves the dead-code errors behind.

Follows the session_get_cookie_params() change in #6004, which added the PHP 8.5 partitioned key the same way.