feat(gateway): wire the wallet.order edge call for the direct rail

Add the WalletOrderRequest / WalletOrderResponse FlatBuffers messages and the
wallet.order Connect op: the gateway decodes the order request, forwards it to
the backend POST /wallet/order and returns the created order id plus the
provider launch URL the client opens. Regenerate the committed Go and TS
FlatBuffers code.

gateway/internal/transcode/encode.go

@@ -37,6 +37,19 @@ func encodeAck(ok bool) []byte {
 	return b.FinishedBytes()
 }
 
+// encodeWalletOrder builds a WalletOrderResponse payload: the created order id and the provider
+// launch URL the client opens.
+func encodeWalletOrder(o backendclient.WalletOrderResp) []byte {
+	b := flatbuffers.NewBuilder(128)
+	oid := b.CreateString(o.OrderID)
+	url := b.CreateString(o.RedirectURL)
+	fb.WalletOrderResponseStart(b)
+	fb.WalletOrderResponseAddOrderId(b, oid)
+	fb.WalletOrderResponseAddRedirectUrl(b, url)
+	b.Finish(fb.WalletOrderResponseEnd(b))
+	return b.FinishedBytes()
+}
+
 // encodeDeleteRequestResult builds an AccountDeleteRequestResult payload reporting which
 // deletion step-up the account uses ("email" | "phrase").
 func encodeDeleteRequestResult(method string) []byte {