+source currency pick fix +fx side propagation

frontend/pshared/test/payment/request_dto_format_test.dart

@@ -7,7 +7,9 @@ import 'package:pshared/api/requests/payment/initiate_payments.dart';
 import 'package:pshared/api/requests/payment/quote.dart';
 import 'package:pshared/api/responses/payment/quotation.dart';
 import 'package:pshared/data/dto/money.dart';
+import 'package:pshared/data/dto/payment/currency_pair.dart';
 import 'package:pshared/data/dto/payment/endpoint.dart';
+import 'package:pshared/data/dto/payment/intent/fx.dart';
 import 'package:pshared/data/dto/payment/intent/payment.dart';
 import 'package:pshared/data/mapper/payment/payment.dart';
 import 'package:pshared/models/payment/asset.dart';
@@ -76,6 +78,36 @@ void main() {
       expect(destination['type'], equals('cardToken'));
     });
 
+    test('quote payment request serializes fx side to backend value', () {
+      final request = QuotePaymentRequest(
+        idempotencyKey: '',
+        previewOnly: true,
+        intent: const PaymentIntentDTO(
+          kind: 'payout',
+          source: PaymentEndpointDTO(
+            type: 'managedWallet',
+            data: {'managed_wallet_ref': 'mw-1'},
+          ),
+          destination: PaymentEndpointDTO(
+            type: 'cardToken',
+            data: {'token': 'tok_1', 'masked_pan': '4111'},
+          ),
+          amount: MoneyDTO(amount: '10', currency: 'USDT'),
+          settlementMode: 'fix_source',
+          fx: FxIntentDTO(
+            pair: CurrencyPairDTO(base: 'RUB', quote: 'USDT'),
+            side: 'buy_base_sell_quote',
+          ),
+        ),
+      );
+
+      final json =
+          jsonDecode(jsonEncode(request.toJson())) as Map<String, dynamic>;
+      final intent = json['intent'] as Map<String, dynamic>;
+      final fx = intent['fx'] as Map<String, dynamic>;
+      expect(fx['side'], equals('buy_base_sell_quote'));
+    });
+
     test('quote response parses backend fx quote pricedAtUnixMs', () {
       final response = PaymentQuoteResponse.fromJson({
         'accessToken': {'token': 'token', 'expiration': '2026-02-25T00:00:00Z'},