Add current pending orders endpoint

[zaixincheng174-ai]

/claim #84

Closes #84

Summary

• add the signed current pending orders endpoint for GET /openApi/swap/v2/trade/openOrders
• expose it through TradeService and the endpoint barrel export
• add focused unit coverage for service dispatch, request path/method, parameter serialization, and optional filter omission
• mark the README current pending orders feature as supported

Validation

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

Disclosure

Prepared with AI assistance and locally validated.