feat(ops): add ignore insufficient balance errors toggle and extract error constants

- Add 5th error filter switch IgnoreInsufficientBalanceErrors to suppress
  upstream insufficient balance / insufficient_quota errors from ops log
- Extract hardcoded error strings into package-level constants for
  shouldSkipOpsErrorLog, normalizeOpsErrorType, classifyOpsPhase, and
  classifyOpsIsBusinessLimited
- Define ErrNoAvailableAccounts sentinel error and replace all
  errors.New("no available accounts") call sites
- Update tests to use require.ErrorIs with the sentinel error

frontend/src/i18n/locales/zh.ts

@@ -4016,6 +4016,8 @@ export default {
         ignoreNoAvailableAccountsHint: '启用后，"No available accounts" 错误将不会写入错误日志（不推荐，这通常是配置问题）。',
         ignoreInvalidApiKeyErrors: '忽略无效 API Key 错误',
         ignoreInvalidApiKeyErrorsHint: '启用后，无效或缺失 API Key 的错误（INVALID_API_KEY、API_KEY_REQUIRED）将不会写入错误日志。',
+        ignoreInsufficientBalanceErrors: '忽略余额不足错误',
+        ignoreInsufficientBalanceErrorsHint: '启用后，上游账号余额不足（Insufficient balance）的错误将不会写入错误日志。',
         autoRefresh: '自动刷新',
         enableAutoRefresh: '启用自动刷新',
         enableAutoRefreshHint: '自动刷新仪表板数据，启用后会定期拉取最新数据。',