createCustomer(overrides = {}) {
        const customerId = `cus_${this.#generateRandomId()}`;
        const stripeCustomer = {
            id: customerId,
            object: 'customer',
            name: 'Test User',
            email: customerId + '@example.com',
            subscriptions: {
                type: 'list',
                data: []
            },
            ...overrides
        };
        this.customers.push(stripeCustomer);
        return stripeCustomer;
    }
Creates a mock Stripe customer.
@param {object} overrides - Optional overrides for the customer object.
@returns {object} - The created Stripe customer object.