Empty string added to the broker order class enum

Broker API

Adds "" (empty string) as an accepted value on the OrderClass enum, alongside the existing simple value. This matches how simple orders are represented in request payloads and trade event streams.

Affected Endpoints

• POST /v1/trading/accounts/{account_id}/orders
• PATCH /v1/trading/accounts/{account_id}/orders/{order_id}
• GET /v1/trading/accounts/{account_id}/orders
• GET /v1/trading/accounts/{account_id}/orders/{order_id}
• GET /v1/trading/accounts/{account_id}/orders:by_client_order_id
• DELETE /v1/trading/accounts/{account_id}/orders
• POST /v1/trading/accounts/{account_id}/orders/estimation
• DELETE /v1/trading/accounts/{account_id}/positions
• DELETE /v1/trading/accounts/{account_id}/positions/{symbol_or_asset_id}
• GET /v1/rebalancing/runs
• GET /v1/rebalancing/runs/{run_id}
• GET /v1/events/trades
• GET /v2/events/trades