wc-composable-product/tests/stubs/class-wc-cart.php

<?php
/**
 * Minimal WC_Cart stub for unit testing.
 */
class WC_Cart {
    protected $cart_contents = [];

    public function get_cart() {
        return $this->cart_contents;
    }

    public function set_cart_contents($contents) {
        $this->cart_contents = $contents;
    }
}