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/backendclient/api.go

@@ -396,6 +396,24 @@ func (c *Client) WalletBuy(ctx context.Context, userID, productID string) (Walle
 	return out, err
 }
 
+// walletOrderBody is the POST body of an order: the chip pack to fund.
+type walletOrderBody struct {
+	ProductID string `json:"product_id"`
+}
+
+// WalletOrderResp is a created order: its id and the provider launch URL the client opens.
+type WalletOrderResp struct {
+	OrderID     string `json:"order_id"`
+	RedirectURL string `json:"redirect_url"`
+}
+
+// WalletOrder opens a pending order to fund a chip pack and returns the provider launch URL.
+func (c *Client) WalletOrder(ctx context.Context, userID, productID string) (WalletOrderResp, error) {
+	var out WalletOrderResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/user/wallet/order", userID, "", walletOrderBody{ProductID: productID}, &out)
+	return out, err
+}
+
 // CatalogAtomResp is one atom line of a storefront product: the value type it grants and quantity.
 type CatalogAtomResp struct {
 	AtomType string `json:"atom_type"`