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

backend/internal/service/openai_account_scheduler.go

@@ -725,7 +725,7 @@ func (s *defaultOpenAIAccountScheduler) selectByLoadBalance(
 		}, len(candidates), topK, loadSkew, nil
 	}
 
-	return nil, len(candidates), topK, loadSkew, errors.New("no available accounts")
+	return nil, len(candidates), topK, loadSkew, ErrNoAvailableAccounts
 }
 
 func (s *defaultOpenAIAccountScheduler) isAccountTransportCompatible(account *Account, requiredTransport OpenAIUpstreamTransport) bool {