fix: fallback to V2 granular scopes on 401/403 as well as invalid_scope

[ErnestHysa]

Problem

New Custom Connections created after the April 2025 deprecation of accounting.transactions fail authentication because the server returns 401 or 403 instead of 400 invalid_scope.

The xero-client only falls back from V1 (deprecated accounting.transactions) to V2 (granular) scopes on an invalid_scope error, but new connections using deprecated V1 scopes receive 401/403 instead.

Fix

Extend the V1→V2 fallback condition to also trigger on 401 and 403 responses, not just on 400 invalid_scope.

const isCredentialError =
  axiosError.response?.status === 401 ||
  axiosError.response?.status === 403;

if (!isInvalidScope && !isCredentialError) {
  throw this.formatTokenError(error, " with V1 scopes");
}

Closes #175