You need to rename your template file to portfolio-shortcode.php
(with a dash rather than an underscore). get_template_part is checking for the following file:
{current_theme}/includes/shortcode_helpers/portfolio-shortcode.php
You also need to change the first parameter of get_template_part to remove the leading slash:
get_template_part('includes/shortcode_helpers/portfolio', 'shortcode');
See get_template_part on the WordPress Codex for the order of the files this function tries to include.