[quality] labelIssue's refused POST /issues/{n}/labels write (src/utils/labeling.ts L88) has no end-to-end coverage #336

Finding

labelIssue (src/utils/labeling.ts L80–89) is the single write path every label command and plugin uses (/kind, /area, /hold, /lgtm, ok-to-test, approve, owners-label, require-matching-label). Its catch at L88 rewraps a refused POST /repos/{o}/{r}/issues/{n}/labels as could not add labels: <cause>, which is what reaches the run's ::error:: annotation.

• Unit evidence: npx vitest run --coverage on main @ 3fc21f2 — labeling.ts 100 % lines; the L88 arm is asserted in __tests__/label/area.test.ts, __tests__/plugins/ownersLabel.test.ts, __tests__/plugins/requireMatchingLabel.test.ts and __tests__/cronJobTest/sweep.test.ts.
• End-to-end evidence: npm run test:coverage:e2e on main @ 3fc21f2 (Node v26.10.0, vitest 5.0.3, 6 files / 142 tests) — labeling.ts 73.07 % lines, uncovered 29–32, 52–59, 88, 114, 135, 170, 174, 203. L88 is never executed: every __tests__/bundle case that reaches a label write routes POST …/issues/1/labels to 200.

Of the other uncovered labeling.ts lines, L29–32 (#246), L135 (#298), L174 (#300/#310/#326) and L202–203 (#326) are claimed by open hold-gated PRs; L52–59 (getArgumentLabels) and L114 (resetLabelCache) have no caller under src/, and L170 is the legacy string-label shape GitHub no longer returns — none of those is reachable through the bundle. L88 is the one remaining reachable arm.

Recommendation

• __tests__/bundle/labelWriteRefused.test.ts (new): /kind cleanup with .prowlabels.yaml served, GET /labels listing kind/cleanup, and POST /issues/1/labels answered 500 → exit 1, the single error error handling issue comment: Error: could not add labels: HttpError: boom, exactly one POST with { labels: ['kind/cleanup'] } after the configuration reads and the labels read.

Measured: with that file, labeling.ts e2e lines 73.07 → 75 % and L88 leaves the uncovered set; bundle suite 142 → 143 tests.

Priority

• Impact: medium — unit-covered, e2e-uncovered (priority 2 under the coverage evidence rules)
• Effort: low

Filed by quality agent (hold-gated mode)