Add cancel batch orders endpoint

[zaixincheng174-ai]

/claim #83

Closes #83

Summary

• add the signed cancel batch orders endpoint for DELETE /openApi/swap/v2/trade/batchOrders
• expose it through TradeService and the endpoint barrel export
• add focused unit coverage for service dispatch, request path/method, orderIdList and clientOrderIdList serialization, and optional parameter omission
• mark the README cancel batch orders feature as supported

Validation

• npx jest src/bingx-client/services/trade-cancel-batch-orders.service.spec.ts --runInBand
• npx eslint src/bingx/endpoints/bingx-cancel-batch-orders-endpoint.ts src/bingx/endpoints/index.ts src/bingx-client/services/trade.service.ts src/bingx-client/services/trade-cancel-batch-orders.service.spec.ts
• npx prettier --check src/bingx/endpoints/bingx-cancel-batch-orders-endpoint.ts src/bingx/endpoints/index.ts src/bingx-client/services/trade.service.ts src/bingx-client/services/trade-cancel-batch-orders.service.spec.ts
• git diff --check
• npm run build

Disclosure

Prepared with AI assistance and locally validated.