feat: add QUOTE_RATE_UNAVAILABLE outgoing transaction failure reason

mintlify/platform-overview/core-concepts/quote-system.mdx

@@ -454,12 +454,16 @@ For cross-currency quotes the exchange rate applies on top of this: the fee is t
     try {
       await executeQuote(quote.id);
     } catch (error) {
-      if (error.code === 'QUOTE_EXPIRED') {
+      if (error.code === 'QUOTE_EXPIRED' || error.code === 'QUOTE_RATE_UNAVAILABLE') {
         quote = await createQuote(quoteParams); // Recreate with fresh rate
         await executeQuote(quote.id);
       }
     }
     ```
+
+    `QUOTE_RATE_UNAVAILABLE` means the quoted rate was refused when the quote was
+    executed, rather than the quote's own expiry window elapsing. Nothing was
+    exchanged in either case, and the recovery is the same: create a new quote.
   </Accordion>
 
   <Accordion title="Monitor quote status via webhooks">

openapi/components/schemas/transactions/OutgoingTransactionFailureReason.yaml

@@ -1,6 +1,7 @@
 type: string
 enum:
   - QUOTE_EXPIRED
+  - QUOTE_RATE_UNAVAILABLE
   - QUOTE_EXECUTION_FAILED
   - FUNDING_AMOUNT_MISMATCH
   - SCA_NOT_COMPLETED
@@ -31,6 +32,7 @@ description: |
   | Reason | Description |
   |--------|-------------|
   | `QUOTE_EXPIRED` | The quote was not executed before its expiry window |
+  | `QUOTE_RATE_UNAVAILABLE` | The quoted exchange rate was no longer available when the quote was executed, so nothing was exchanged. Create a new quote to get a current rate |
   | `QUOTE_EXECUTION_FAILED` | The quote could not be executed. Covers any internal failure on the way to settlement, whether or not funds were debited — a debited amount is refunded automatically |
   | `FUNDING_AMOUNT_MISMATCH` | The funds received did not match the expected amount |
   | `SCA_NOT_COMPLETED` | The customer did not complete the Strong Customer Authentication challenge before it expired |