[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/CONTRIBUTORS.rst

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+* `Criptomart <https://criptomart.net>`_:
+
+  * Criptomart

pos_payment_method_cashdro_fix/readme/DESCRIPTION.rst

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+The 18.0 migration of ``pos_payment_method_cashdro``
+(`OCA/pos@6a4c251d <https://github.com/OCA/pos/commit/6a4c251d>`_) left three
+uses of the 17.0 API that stop the CashDro integration from working. The module
+has no tests nor tours and needs a physical drawer to be exercised, so nothing
+in the OCA CI catches them.
+
+This module patches the three of them from the outside, without touching the
+OCA sources:
+
+#. ``_cashdro_url()`` reads ``order.selected_paymentline``, replaced in 18.0 by
+   ``order.get_selected_paymentline()``. Reading ``.payment_method`` on the
+   resulting ``undefined`` raises a ``TypeError`` on every payment, which the
+   caller swallows into a generic *"An error occurred while connecting to the
+   cashdro"* message.
+#. The ``add_paymentline`` patch checks ``line.payment_method``, renamed to
+   ``line.payment_method_id`` in 18.0. The condition is never true, so the
+   payment line keeps the default due amount instead of waiting for the amount
+   the customer inserts in the drawer.
+#. ``pos.session._loader_params_pos_payment_method`` is the 17.0 loading API,
+   replaced in 18.0 by ``_load_pos_data_fields`` on each model. The override is
+   dead code, so ``cashdro_host``, ``cashdro_user`` and ``cashdro_password``
+   never reach the front end.

pos_payment_method_cashdro_fix/readme/ROADMAP.rst

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+This module is meant to be temporary. The fixes are proposed upstream in
+``upstream/pos_payment_method_cashdro-18.0-fixes.patch``; once they are merged
+in ``OCA/pos`` this module can be uninstalled and removed.
+
+It is written to stay harmless in the meantime: the Python override only adds
+the credential fields that are missing from the loader, and the JavaScript
+patches are idempotent with respect to the fixed upstream code.
+
+The JavaScript side can't be covered by automated tests, as it needs a CashDro
+terminal answering on the network. See ``USAGE`` for the manual check.

pos_payment_method_cashdro_fix/readme/USAGE.rst

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+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