ci: runner outage leaves PRs indefinitely pending with a required context that never reports — reads as '19 success, 0 failures' #13045

Sibling of #12823. That issue covers PRs left with zero checks because GITHUB_TOKEN bot pushes do not trigger workflows. This is a second, distinct route to the same dangerous end state: a PR that reads as green but is structurally unmergeable.

Observed 2026-07-28 on PRs #12895 and #12896

While the singleton self-hosted runner was offline:

runner:  Little-Slave  status=offline  busy=false  labels=self-hosted,Linux,X64
run:     status=pending  conclusion=null  jobs=[]        <- zero jobs ever created

Both PRs reported 19 success / 0 failures across every check that ran, while being blocked the whole time: the required context Unit & Integration Tests had produced no check-run at all, so commits/{sha}/status sat at pending.

Three properties made this hard to diagnose, and each is independently worth guarding against:

1. Absence is not failure. Any tooling that summarises success/failure counts reports these PRs as clean. The only way to see the block is to diff the reported contexts against branches/<base>/protection.required_status_checks.contexts.
2. The approval sweep does not catch it. The standard remedy searches for conclusion == "action_required". These runs are status=pending, conclusion=null, and POST /actions/runs/{id}/approve returns 403 not waiting for approval.
3. Runs created during the outage never self-schedule. After the runner came back, both runs went to completed/cancelled with jobs: 0. gh run rerun cannot help a run that never created jobs — it has nothing to re-run.

frontend-test.yml is correctly written for the ordinary case: it deliberately carries no pull_request.paths filter and ships a unit-tests-skip shim (same name:, ubuntu-latest) precisely so the required context always reports. That design is defeated when the runner hosting the real job is offline, because the run never reaches the point of choosing a branch.

Scope

• Give the required-context shim a path that does not depend on the self-hosted runner, so the context reports even when the runner pool is empty.
• Or: surface the condition — a check that compares reported contexts against the required list and fails loudly with "required context X never reported", instead of leaving the PR silently at pending.

Done when

• A runner outage produces a visible, named failure on affected PRs rather than an indefinite pending with no explanation.
• The diagnosis does not require manually diffing check-runs against branch protection.

Refs #12823