Add account profit/loss fund flow endpoint

[zaixincheng174-ai]

/claim #78

Closes #78

Summary

• add the signed account profit and loss fund flow endpoint for GET /openApi/swap/v2/user/income
• expose it through AccountService and the endpoint barrel export
• add focused unit coverage for service dispatch, request path/method, parameter serialization, and optional filter omission
• mark the README account-income feature as supported

Validation

• npx jest src/bingx-client/services/account-income.service.spec.ts --runInBand
• npx eslint src/bingx/endpoints/bingx-account-profit-loss-fund-flow-endpoint.ts src/bingx/endpoints/index.ts src/bingx-client/services/account.service.ts src/bingx-client/services/account-income.service.spec.ts
• npx prettier --check src/bingx/endpoints/bingx-account-profit-loss-fund-flow-endpoint.ts src/bingx/endpoints/index.ts src/bingx-client/services/account.service.ts src/bingx-client/services/account-income.service.spec.ts
• git diff --check
• npm run build

Disclosure

Prepared with AI assistance and locally validated.