[FIX] payment: provide missing return_url in values

Render from payment//payment_acquirer.py promises:
All templates will receive:
    'feedback_url', 'return_url', 'cancel_url', 'error_url'
However they are all in -> FIXME.
After 6 years in FIXME, this promise is still a lie.

It is unfortunate since the Adyen payment provider requires 'return_url' to work.
Before, this function was called by _get_shop_payment_values,
which provided a value for it.
We copy the value that was given there directly as a default in render,
so that it doesn't crash no matter what method calls it.

opw 1928918
opw 1944142

closes odoo/odoo#31814

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/payment/models/payment_acquirer.py

@@ -356,6 +356,7 @@ class PaymentAcquirer(models.Model):
         if values is None:
             values = {}
 
+        values.setdefault('return_url', '/payment/process')
         # reference and amount
         values.setdefault('reference', reference)
         amount = float_round(amount, 2)
@@ -447,7 +448,6 @@ class PaymentAcquirer(models.Model):
             'context': self._context,
             'type': values.get('type') or 'form',
         })
-        values.setdefault('return_url', False)
 
         _logger.info('payment.acquirer.render: <%s> values rendered for form payment:\n%s', self.provider, pprint.pformat(values))
         return self.view_template_id.render(values, engine='ir.qweb')