Wrong serialization of TriggerBy enum

[wellfour]

The TriggerBy enum holds correct API values ("LastPrice", "MarkPrice", "IndexPrice"), but serialization currently uses name() (LAST_PRICE, MARK_PRICE, INDEX_PRICE).

Because of this, orders are created but triggerBy is ignored (e.g. TP/SL conditions are not applied correctly).

Expected: Serialize TriggerBy using the API value.
Fix: Override toString() to return trigger, or ensure DTOs use getTrigger() instead of name().