[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/tests/__init__.py

@@ -0,0 +1 @@
+from . import test_load_pos_data_fields

pos_payment_method_cashdro_fix/tests/test_load_pos_data_fields.py

@@ -0,0 +1,33 @@
+# Copyright 2026 - Today Criptomart
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
+
+from odoo.tests import tagged
+from odoo.tests.common import TransactionCase
+
+from ..models.pos_payment_method import CASHDRO_FIELDS
+
+
+@tagged("post_install", "-at_install")
+class TestLoadPosDataFields(TransactionCase):
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+        cls.payment_method = cls.env["pos.payment.method"]
+        # Nothing dereferences it, but we pass a real config when there's one
+        cls.config_id = cls.env["pos.config"].search([], limit=1).id
+
+    def test_cashdro_credentials_are_loaded(self):
+        """The front end can't build the CashDro url without them."""
+        fields = self.payment_method._load_pos_data_fields(self.config_id)
+        for field in CASHDRO_FIELDS:
+            self.assertIn(field, fields)
+
+    def test_no_duplicated_fields(self):
+        """Once the base module is fixed upstream we must not add them twice."""
+        fields = self.payment_method._load_pos_data_fields(self.config_id)
+        self.assertEqual(sorted(fields), sorted(set(fields)))
+
+    def test_fields_exist_on_the_model(self):
+        """Guard against a rename of the credentials in the base module."""
+        for field in CASHDRO_FIELDS:
+            self.assertIn(field, self.payment_method._fields)