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/ops_retry.go

@@ -467,7 +467,7 @@ func (s *OpsService) executeClientRetry(ctx context.Context, reqType opsRetryReq
 			return &opsRetryExecution{status: opsRetryStatusFailed, errorMessage: selErr.Error()}
 		}
 		if selection == nil || selection.Account == nil {
-			return &opsRetryExecution{status: opsRetryStatusFailed, errorMessage: "no available accounts"}
+			return &opsRetryExecution{status: opsRetryStatusFailed, errorMessage: ErrNoAvailableAccounts.Error()}
 		}
 
 		account := selection.Account