test: close tea and error-classifier coverage gaps, raise thresholds

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.

vitest.config.ts

@@ -23,10 +23,10 @@ export default defineConfig({
       // coverage so a real drop fails CI while trivial churn does not. Raise
       // these as coverage climbs; never lower them to make a red build pass.
       thresholds: {
-        statements: 85,
-        branches: 78,
-        functions: 90,
-        lines: 85,
+        statements: 92,
+        branches: 87,
+        functions: 95,
+        lines: 92,
       },
     },
   },