[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/CONTRIBUTORS.rst

@@ -0,0 +1,3 @@
+* `Criptomart <https://criptomart.net>`_:
+
+  * Criptomart