[ADD] pos_payment_method_cashdro_fix: 17.0 API leftovers in the OCA base module

The 18.0 migration of `pos_payment_method_cashdro` (OCA/pos@6a4c251d) kept
three uses of APIs removed in 18.0. The module has no tests nor tours and needs
a physical drawer to be exercised, so the OCA CI can't catch them:

* `_cashdro_url()` reads `order.selected_paymentline`, so `.payment_method` on
  the resulting `undefined` throws a TypeError on every payment, swallowed into
  a generic "An error occurred while connecting to the cashdro" dialog.
* The `add_paymentline()` patch checks `line.payment_method`, renamed to
  `payment_method_id`, so the line keeps the default due amount instead of
  waiting for the amount the customer inserts.
* `_loader_params_pos_payment_method()` is the 17.0 loading API, replaced by
  `_load_pos_data_fields()`. Dead code, so the CashDro credentials never reach
  the front end.

Patched from the outside, as OCA sources are not to be modified. Written to
stay harmless once fixed upstream: the Python override only adds the missing
fields and the JS patches are idempotent against the fixed code.

`upstream/pos_payment_method_cashdro-18.0-fixes.patch` holds the same fixes as
a ready to send `git format-patch`, verified with `git apply --check`. This
module is temporary and should be removed once that lands in OCA/pos.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pos_payment_method_cashdro_fix/static/src/js/models_fix.esm.js

@@ -0,0 +1,31 @@
+/* Copyright 2026 - Today Criptomart
+   License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
+
+   The `add_paymentline` patch of `pos_payment_method_cashdro` checks
+   `line.payment_method`, renamed to `payment_method_id` in 18.0, so the
+   condition is never met and the line keeps the default due amount instead of
+   waiting for the amount the customer actually inserts in the drawer.
+*/
+
+import { PosOrder } from "@point_of_sale/app/models/pos_order";
+import { patch } from "@web/core/utils/patch";
+
+patch(PosOrder.prototype, {
+    /**
+     * @override
+     * Set the amount to 0 as it's going to be filled by the Cashdro response.
+     */
+    add_paymentline() {
+        const line = super.add_paymentline(...arguments);
+        if (!line) {
+            return line;
+        }
+        if (
+            line.payment_method_id?.use_payment_terminal === "cashdro" &&
+            line.amount > 0 // For refund transactions we need to keep the amount
+        ) {
+            line.set_amount(0);
+        }
+        return line;
+    },
+});

pos_payment_method_cashdro_fix/static/src/js/payment_cashdro_fix.esm.js

@@ -0,0 +1,33 @@
+/* Copyright 2026 - Today Criptomart
+   License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl).
+
+   `_cashdro_url` was left with the 17.0 API in the 18.0 migration of
+   `pos_payment_method_cashdro`: `order.selected_paymentline` no longer exists,
+   so reading `.payment_method` on it throws a TypeError that the caller
+   swallows as a generic connection error. Rewritten with the 18.0 accessors.
+*/
+
+import { PaymentCashdro } from "@pos_payment_method_cashdro/js/payment_cashdro.esm";
+import { patch } from "@web/core/utils/patch";
+
+patch(PaymentCashdro.prototype, {
+    /**
+     * @override
+     * The whole body has to be replaced: the very first statement is the one
+     * that breaks, so there is nothing left to delegate to `super`.
+     */
+    _cashdro_url() {
+        // Cashdro machines don't support safe POST calls, so we're sending
+        // all the data quite unsafely constantly...
+        const order = this.pos.get_order();
+        const method = order.get_selected_paymentline()?.payment_method_id;
+        const host = method && method.cashdro_host;
+        if (!host) {
+            return false;
+        }
+        let url = `${host}/Cashdro3WS/index.php`;
+        url += `?name=${method.cashdro_user}`;
+        url += `&password=${method.cashdro_password}`;
+        return url;
+    },
+});