Add PaymentRail enum for external accounts (#347)

## Summary
- Introduces a new `PaymentRail` enum in `openapi/components/schemas/common/PaymentRail.yaml` containing all 14 payment rails used across external accounts: ACH, BANK_TRANSFER, FAST, FASTER_PAYMENTS, FEDNOW, MOBILE_MONEY, PAYNOW, PIX, RTP, SEPA, SEPA_INSTANT, SPEI, UPI, WIRE
- Wires the enum into `AccountDestination.paymentRail` (quotes) and `ExchangeRate.destinationPaymentRail` (exchange rates) which previously used untyped strings
- Individual per-currency `AccountInfo` schemas retain their currency-specific subset enums for stricter validation

## Test plan
- [x] `make build` succeeds — OpenAPI spec bundles correctly
- [x] `npx @redocly/cli lint openapi.yaml` passes validation (16 pre-existing warnings, 0 errors)
- [ ] Verify generated SDK clients pick up the new enum type
- [ ] Verify Mintlify docs render the enum values on API reference pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: AaryamanBhute

mintlify/openapi.yaml

@@ -0,0 +1,20 @@
+type: string
+enum:
+  - ACH
+  - BANK_TRANSFER
+  - FAST
+  - FASTER_PAYMENTS
+  - FEDNOW
+  - MOBILE_MONEY
+  - PAYNOW
+  - PIX
+  - RTP
+  - SEPA
+  - SEPA_INSTANT
+  - SPEI
+  - UPI
+  - WIRE
+description: >-
+  The payment rail used for the transfer. Payment rails represent the underlying
+  payment network or system used to move funds between accounts.
+example: ACH

openapi.yaml

@@ -35,9 +35,10 @@ properties:
   destinationCurrency:
     $ref: ../common/Currency.yaml
   destinationPaymentRail:
-    type: string
-    description: The payment rail used for the destination (e.g., UPI, SEPA_INSTANT, MOBILE_MONEY, FASTER_PAYMENTS)
-    example: UPI
+    allOf:
+      - $ref: ../common/PaymentRail.yaml
+      - description: The payment rail used for the destination (e.g., UPI, SEPA_INSTANT, MOBILE_MONEY, FASTER_PAYMENTS)
+        example: UPI
   receivingAmount:
     type: integer
     format: int64

openapi/components/schemas/common/PaymentRail.yaml

@@ -15,10 +15,10 @@ allOf:
         description: Destination account identifier
         example: ExternalAccount:a12dcbd6-dced-4ec4-b756-3c3a9ea3d123
       paymentRail:
-        type: string
-        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.
-        example: ACH
+        allOf:
+          - $ref: ../common/PaymentRail.yaml
+          - 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.
     description: Destination account details