feat: let production select Mercado Pago and acknowledge simulated notifications

The production compose hard-coded the fake payment adapter; it now takes
PAYMENT_ADAPTER and the MP_* settings from the stack's environment, so the
sandbox can run with test credentials. A signed notification about a payment
Mercado Pago does not have, such as the panel's "Simular notificação", is
acknowledged instead of answering 500 and being retried; any other lookup
failure still raises.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_mercadopago.py

@@ -36,6 +36,10 @@ class MercadoPagoTests(unittest.TestCase):
             self.requests.append(request)
             if request.method == 'GET':
                 payment_id = request.url.path.rsplit('/', 1)[-1]
+                if payment_id == '500':
+                    return httpx.Response(500, json={'message': 'internal_error'})
+                if payment_id not in self.payments:
+                    return httpx.Response(404, json={'message': 'Payment not found'})
                 return httpx.Response(200, json=self.payments[payment_id])
             body = json.loads(request.content)
             payment = {'id': 555, 'status': 'pending', 'status_detail': 'pending_waiting_transfer',
@@ -78,6 +82,15 @@ class MercadoPagoTests(unittest.TestCase):
         self.assertEqual(self.requests[-1].headers['authorization'], 'Bearer TEST-token')
         self.assertIsNone(self.mp.parse(json.dumps({'type': 'merchant_order', 'data': {'id': '1'}}).encode()))
 
+    def test_notification_for_an_unknown_payment_is_acknowledged(self):
+        # The panel's "Simular notificação" sends a payment id that does not
+        # exist. Raising would answer 500 and Mercado Pago would retry for ever.
+        body = json.dumps({'id': 43, 'type': 'payment', 'data': {'id': '123456'}}).encode()
+        self.assertIsNone(self.mp.parse(body, {'data.id': '123456', 'type': 'payment'}))
+        # Any other failure still raises, so a real notification is retried.
+        with self.assertRaises(httpx.HTTPStatusError):
+            self.mp.parse(json.dumps({'type': 'payment', 'data': {'id': '500'}}).encode(), {'data.id': '500'})
+
     def test_amounts_outside_brl_centavos_are_not_trusted(self):
         for payment in ({'currency_id': 'USD', 'transaction_amount': 10},
                         {'currency_id': 'BRL', 'transaction_amount': 10.001},