Fix issue oncall schedule expiry

[cortinico]

Summary:

The scheduled Monitor React Native New Issues workflow fails in Extract next oncall because extractIssueOncalls.js requires an exact dated entry and the ONCALL_SCHEDULE secret has no entry for the active Tuesday.

Live evidence:

• Affected default-branch head: 99d327c613cdd4c3614cc3baa286a2d57a629136
• Current failed run: https://github.com/react/react-native/actions/runs/37395939025
• Error: TypeError: Cannot read properties of undefined (reading '0') at schedule[dateStr][0]
• Last successful scheduled run: https://github.com/react/react-native/actions/runs/20724738462 on 2026-01-05
• First failed scheduled run: https://github.com/react/react-native/actions/runs/20733701571 on 2026-01-06
• No matching open PR was found before preparing this change.

The exact-date lookup was introduced in 890b8f7ea3378685f7d6d9612ff584fc9ed17df0. The outage itself was triggered by the schedule crossing its final configured date, not by a new January commit, so a code bisection cannot isolate a later introducing commit.

This change treats each dated rotation as effective until the next dated entry, which keeps issue monitoring alive when schedule maintenance is late. It writes a warning to stderr whenever it carries a rotation forward, so the stale configuration remains visible. Dates before the first configured entry still fail clearly.

Risk / limitation: until the secret is refreshed, the last configured pair remains on call. That is preferable to the current complete notification outage, but schedule owners should still update the secret.

Changelog:

[INTERNAL] [FIXED] - Keep issue monitoring running when the on-call schedule reaches its final dated entry.

Test Plan:

• npx --yes jest@29.7.0 --runInBand --config '{"testEnvironment":"node","rootDir":"/tmp/react-native-ci-monitor/repo","transform":{}}' .github/workflow-scripts/__tests__/extractIssueOncalls-test.js — 9/9 tests passed, including the new expired-schedule regression case.
• npx --yes prettier@3.9.4 --check .github/workflow-scripts/extractIssueOncalls.js .github/workflow-scripts/__tests__/extractIssueOncalls-test.js — passed.
• node --check on both changed files — passed.
• git diff --check — passed.
• Monitor React Native New Issues workflow-dispatch run on this branch — passed end to end: https://github.com/react/react-native/actions/runs/37396984698. The real secret took the carry-forward path, emitted the expected warning, and completed Monitor New Issues successfully.

A full workspace install was not run locally because the sparse diagnostic checkout cannot resolve unpublished 0.87.0-main workspace packages from npm; GitHub PR CI will exercise the repository-wide checks.