Cover the previously untested credential and error-classification paths:
- tea.ts: failing/invalid/non-array `tea login list`, a failing token
helper, and an empty token, all driven through the CLI seam via the fake
tea harness (now parametrizable for exit codes and output).
- errors.ts: a unit-tier test of classifyHttpError for the pure-classifier
branches no shipped command reaches yet (issue/pull 404s, the
already-classified passthrough, transport failures, no-body defaults).
errors.ts is now fully covered and tea.ts is at 96% (the remainder is
defensive code unreachable through the seam). Overall coverage rose from
~87% to ~93% statements / ~89% branches; the ratchet is raised to
92/87/95/92 accordingly.
