fix: let Mercado Pago take the card issuer from the card number

Paying with Mercado Pago's own test card failed with 10111 ("the issuer
does not have the BIN configured"): the issuer_id suggested by the card
form did not match the card. issuer_id is optional, and without it Mercado
Pago resolves the issuer from the BIN.

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

app/mercadopago.py

@@ -85,8 +85,9 @@ class MercadoPagoPayment:
             body.update(token=method['token'], payment_method_id=method['payment_method_id'],
                         installments=int(method.get('installments', 1)),
                         three_d_secure_mode='optional')
-            if method.get('issuer_id'):
-                body['issuer_id'] = method['issuer_id']
+            # No issuer_id: Mercado Pago takes the issuer from the card number.
+            # The form's suggestion was refused for its own test cards
+            # (10111, "the issuer does not have the BIN configured").
         else:
             raise ValueError('Unsupported payment method')
         # A PIX retry must return the same code; a new one, after the last