[ADD] pos_payment_method_cashdro_flow_fix: fix the CashDro payment flow on 18.0

`_cashdro_payment_url()` builds the startOperation url with
`this.pos.pos_session.name`. 18.0 replaced that accessor by the `session`
getter of PosStore, so reading `.name` on the resulting `undefined` raises a
TypeError on every single payment, before any request leaves the browser.
`cashdro_send_payment_request` catches it with a bare `catch` that neither
logs nor rethrows and shows the generic "error connecting to the cashdro",
which is why the failure looked like a problem with the machine.

`pos_payment_method_cashdro_fix` only covers the data loading, so this goes
in a module of its own: it has a longer life than that temporary patch, and
it supersedes its `_cashdro_url()` override.

Along with the blocking one, the flow gets:

* the real error logged and shown in the dialog, so a wrong host, a busy
  drawer and a dead network stop being the same sentence,
* a missing host raised instead of the boolean `false` travelling into
  `fetch`, where it resolves against the Odoo server,
* `finishOperation` always sent on failure, or the drawer keeps the operation
  open and refuses every later payment of the session,
* polling on a 700ms interval with a 5 minute deadline, stopped by a cancel
  or by leaving the payment screen, instead of a tight unbounded loop,
* `send_payment_cancel()` returning whether the drawer took it, so the line
  no longer sticks in `waitingCard`,
* reversals sending the amount of the line negated: `get_due()` only
  discounts the payments already done, so the base module asked the drawer to
  collect the money a second time instead of dispensing it,
* the `uuid` of the line honoured instead of the selected one,
* the CashDro answers validated (HTTP status, JSON body, operation id).

`_cashdro_amount()` is split out as the single hook for the amount, which
`pos_payment_method_cashdro_rounding` uses.

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

pos_payment_method_cashdro_flow_fix/readme/USAGE.rst

@@ -0,0 +1,53 @@
+There is nothing to configure: installing the module is enough.
+
+Before anything else, check the payment method
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+18.0 added the ``payment_method_type`` field (*Integration*) to
+``pos.payment.method``, and core clears ``use_payment_terminal`` whenever it is
+saved as *None required*. On **Point of Sale ‣ Configuration ‣ Payment
+Methods**, the CashDro method must have:
+
+* **Integration** = *Terminal*
+* **Integrate with** = *Cashdro*
+* the terminal host, user and password filled in
+
+With *Integration* left at *None required* the terminal is not wired at all:
+clicking the payment method just adds a plain cash line, and nothing is ever
+sent to the drawer — no dialog, no error, no traffic.
+
+On site
+~~~~~~~
+
+#. 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 (that one comes from ``pos_payment_method_cashdro_fix``).
+#. The drawer must start the operation and ask for the amount.
+
+If it still fails, the dialog now carries the reason instead of the generic
+sentence, and the browser console has the full error under ``[CASHDRO]``. What
+to expect:
+
+``The CashDro payment method '...' has no terminal host configured.``
+    The credentials did not reach the front end. Either the fields are empty on
+    the payment method, or ``pos_payment_method_cashdro_fix`` is not installed.
+
+``TypeError: Failed to fetch`` / ``NetworkError``
+    The browser cannot reach ``cashdro_host``. Check the host and that the till
+    is on the same network as the drawer. A PoS served over HTTPS also blocks a
+    plain ``http://`` drawer as mixed content, which shows up here.
+
+``The CashDro answered 401/403/500 ...``
+    The drawer is reachable and answering: wrong user or password, or the
+    CashDro3WS service is failing.
+
+``The CashDro did not return an operation id: ...``
+    The drawer refused ``startOperation``. Its own envelope is in the message —
+    usually a busy drawer with an operation still open from a previous failure.
+
+``The CashDro did not finish the operation in time.``
+    ``startOperation`` was accepted but the drawer never reached the finished
+    state within five minutes.
+
+There are no automated tests: the flow needs a CashDro terminal answering on the
+network, which is also why none of this is caught by the OCA CI.