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/api/admin/ops.ts

@@ -841,6 +841,7 @@ export interface OpsAdvancedSettings {
   ignore_context_canceled: boolean
   ignore_no_available_accounts: boolean
   ignore_invalid_api_key_errors: boolean
+  ignore_insufficient_balance_errors: boolean
   display_openai_token_stats: boolean
   display_alert_events: boolean
   auto_refresh_enabled: boolean