[FIX] payment: initialize txContext before await

Steps to reproduce:
- install website_sale;
- install website_sale_picking;
- enable just "Pay in store when picking the product" provider;
- pusblish just "[On Site Pick] My Shop 1" shipping method;
- go to ecommerce and make the purchase flow.

Issue:
A traceback appears on the "/shop/payment" page.

Cause:
We use the `txContext` field in the `_setPaymentFlow`
method before it is initialized.
Indeed during the `start` method, we have to wait for the end
of the super before `txContext` is initialized.
Since the `start` method is asynchronous, this will not block
the `_setPaymentFlow` method call which will use the uninitialized field.

opw-3257641

closes odoo/odoo#119351

X-original-commit: 18b629bfc26ab69e5e3bdc8aac090de66cf13f36
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>
Signed-off-by: Lefebvre Thomas (thle) <thle@odoo.com>

addons/payment/static/src/js/payment_form_mixin.js

@@ -31,6 +31,8 @@
          * @override
          */
         start: async function () {
+            this.txContext = {}; // Synchronously initialize txContext before any await.
+            Object.assign(this.txContext, this.$el.data());
             await this._super(...arguments);
             window.addEventListener('pageshow', function (event) {
                 if (event.persisted) {
@@ -38,8 +40,6 @@
                 }
             });
             this.$('[data-bs-toggle="tooltip"]').tooltip();
-            this.txContext = {};
-            Object.assign(this.txContext, this.$el.data());
             const $checkedRadios = this.$('input[name="o_payment_radio"]:checked');
             if ($checkedRadios.length === 1) {
                 const checkedRadio = $checkedRadios[0];