feat(sbp_payments): added tests for new chosen_plan_state fixtures and new tests for chat repo and service

test/features/chat/models/payment_model_test.dart

@@ -33,5 +33,15 @@ void main() {
         expect(model.paymentProvider, 'alfa-bank');
       },
     );
+
+    test('fromJson парсит mock-ответ (checkout_url без order_id)', () {
+      final json = FixtureReader.readAsMap('chat/payments/payment_response.json');
+
+      final model = PaymentModel.fromJson(json);
+
+      expect(model.paymentUrl, 'https://example.com/checkout/payment-123');
+      expect(model.orderId, isNull);
+      expect(model.paymentProvider, isNull);
+    });
   });
 }