New "group_id" and "group_role" fields on orders

Adds two optional fields to the Order and OrderLeg schemas so callers can identify which bracket, OTO, or OCO order an individual order belongs to:

• group_id: an opaque UUID shared by every order in the same bracket, oto, or oco order, including legs and replacements.
• group_role: one of entry, take_profit, or stop_loss from the new OrderGroupRole enum. For oco orders, the top-level order is take_profit.

Both keys are omitted for orders that are not part of a bracket, oto, or oco order. New bracket-order response examples have been added.

Broker API

Trade events delivered over SSE carry group_id and group_role on the order payload, so stream-only clients can group legs that arrive individually with legs: null. A bracket-leg trade update example has been added.

Affected Endpoints

• POST /v1/trading/accounts/{account_id}/orders
• 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
• PATCH /v1/trading/accounts/{account_id}/orders/{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 /v2/events/trades

Trading API

• POST /v2/orders
• GET /v2/orders
• GET /v2/orders/{order_id}
• GET /v2/orders:by_client_order_id
• PATCH /v2/orders/{order_id}
• DELETE /v2/positions
• DELETE /v2/positions/{symbol_or_asset_id}