Add account configuration update websocket event

[zaixincheng174-ai]

/claim #93

Closes #93

Summary

• add typed account configuration update websocket event support for ACCOUNT_CONFIG_UPDATE
• expose accountConfigurationUpdate$ on BingxAccountSocketStream
• add focused websocket coverage for parsing leverage and margin-mode configuration updates
• mark the README account configuration update websocket feature as supported

Validation

• npx jest src/bingx-socket/bingx-account-socket-config-update.spec.ts --runInBand
• npx eslint src/bingx-socket/bingx-account-socket-stream.ts src/bingx-socket/events/account-websocket-events.ts src/bingx-socket/bingx-account-socket-config-update.spec.ts
• npx prettier --check src/bingx-socket/bingx-account-socket-stream.ts src/bingx-socket/events/account-websocket-events.ts src/bingx-socket/bingx-account-socket-config-update.spec.ts
• git diff --check
• npm run build

Disclosure

Prepared with AI assistance and locally validated.