Order side enum narrowed to "buy" and "sell"

Broker API

The OrderSide schema now documents only buy and sell, matching the values the API actually accepts and returns.

• Removes buy_minus, sell_plus, sell_short, sell_short_exempt, undisclosed, cross, and cross_short from OrderSide.
• Trade activities now use a separate TradeActivitySide schema, which keeps the full set of side values reported on trade activity records.

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
• DELETE /v1/trading/accounts/{account_id}/positions
• DELETE /v1/trading/accounts/{account_id}/positions/{symbol_or_asset_id}
• GET /v2/events/trades
• GET /v1/accounts/activities
• GET /v1/accounts/activities/{activity_type}