feat(transfers): allow selecting payment rail on transfer-out destination

Add an optional `paymentRail` field to the transfer-out destination,
mirroring the rail selection already available on quote destinations.
Introduces a dedicated `ExternalAccountDestinationReference` schema
(accountId + paymentRail) so the shared `ExternalAccountReference` used by
transfer-in is left unchanged. The field reuses the common `PaymentRail` enum.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: pengying

mintlify/openapi.yaml

@@ -0,0 +1,15 @@
+type: object
+required:
+  - accountId
+properties:
+  accountId:
+    type: string
+    description: Reference to an external account ID
+    example: ExternalAccount:e85dcbd6-dced-4ec4-b756-3c3a9ea3d965
+  paymentRail:
+    description: >-
+      The payment rail to use for the transfer. Must be one of the rails
+      supported by the destination account. If not specified, the system
+      will select a default rail.
+    allOf:
+      - $ref: ../common/PaymentRail.yaml

openapi.yaml

@@ -7,7 +7,7 @@ properties:
     $ref: ./InternalAccountReference.yaml
     description: Source internal account details
   destination:
-    $ref: ./ExternalAccountReference.yaml
+    $ref: ./ExternalAccountDestinationReference.yaml
     description: Destination external account details
   amount:
     type: integer

openapi/components/schemas/transfers/ExternalAccountDestinationReference.yaml

@@ -31,6 +31,7 @@ post:
                 accountId: InternalAccount:a12dcbd6-dced-4ec4-b756-3c3a9ea3d123
               destination:
                 accountId: ExternalAccount:e85dcbd6-dced-4ec4-b756-3c3a9ea3d965
+                paymentRail: ACH
               amount: 12550
   responses:
     '201':