feat(account): implement user fee rate endpoint (#80)

[ZainKazmiii]

Implements #80 (Price: 10$) by wiring the missing account feature end-to-end.

What changed

• Added BingxUserFeeRateEndpoint for GET /openApi/swap/v2/user/commissionRate.
• Added AccountService.getUserFeeRate(account, symbol?) that dispatches the new endpoint.
• Exported the endpoint in src/bingx/endpoints/index.ts.
• Added focused unit tests in src/bingx-client/services/account.service.spec.ts for dispatch and optional symbol forwarding.
• Updated README feature status for User fee rate to checked.

Validation

Validated on the current PR head (a03308846e492e312e1d7dbcceeb19b9cdbcd9df) in a clean checkout:

• npm install --no-audit --no-fund PASS
• npx tsc --noEmit PASS
• npm run build --silent PASS
• npx jest src/bingx-client/services/account.service.spec.ts --runInBand PASS (2 tests)

[ZainKazmiii]

Validation update from clean PR-head rerun:

• Checkout: ZainKazmiii/bingx-api branch fix/80-user-fee-rate-endpoint-20260521 at a03308846e492e312e1d7dbcceeb19b9cdbcd9df
• npm install --no-audit --no-fund -> PASS_AFTER_RETRY (first run hit spawn EPERM, immediate rerun passed)
• npx tsc --noEmit -> PASS
• npm run build --silent -> PASS
• npx jest src/bingx-client/services/account.service.spec.ts --runInBand -> PASS (2 tests)

This supersedes the earlier runner note that dependencies could not be installed.