[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/readme/USAGE.rst

@@ -0,0 +1,21 @@
+There is nothing to configure: installing the module is enough.
+
+To check the three fixes on a PoS with a CashDro terminal:
+
+#. Open a PoS session, add a product and go to the payment screen.
+#. Select the CashDro payment method. The payment line must show **0**, not the
+   due amount (fix 2).
+#. The drawer must start the operation and ask for the amount, instead of the
+   *"An error occurred while connecting to the cashdro"* dialog (fixes 1 and 3).
+#. With no CashDro at hand, the browser console tells fix 1 from fix 3 apart:
+   without this module the failure is a ``TypeError: Cannot read properties of
+   undefined (reading 'payment_method')``; with it, a network error against the
+   configured host.
+
+The Python side is covered by the tests. The base module lives outside this
+repo, so the OCA sources have to be on the addons path (see
+``docker-compose.override.yml``, which is local and untracked)::
+
+  docker-compose run --rm odoo odoo -c /etc/odoo/odoo.conf \
+      --addons-path=/mnt/extra-addons,/mnt/oca/pos,/usr/lib/python3/dist-packages/odoo/addons \
+      -d odoo -u pos_payment_method_cashdro_fix --test-enable --stop-after-init