[CRCR] Failed-test drill-down from RFC-0054 is absent end-to-end (`failed_tests_json` is never populated)

[atalman]

What the RFC specifies

RFC-0054 — Page 2: Per-Backend Dashboard (full doc):

Failure drill-down: click a red cell to see failed test details (parsed from failed_tests_json)

The RFC's Unresolved Question #1 was closed by choosing the storage shape for this feature:

Failed test detail storage Resolved: Embedded as failed_tests_json String column in both DynamoDB and ClickHouse.

Current behaviour

The storage decision landed; the feature never did. It is missing at every stage of the pipeline, not just the UI:

1. No way for a downstream repo to report it. .github/actions/cross-repo-ci-relay-callback/action.yml has nine inputs — status, conclusion, delivery-id, event-type, test-results, callback-url, artifact-url, max-time, job-name. test-results carries only the counts (passed/failed/skipped). There is no input for per-test failure detail.
2. Not extracted. torchci/lib/crcr/crcrUtils.ts → extractDynamoRecord() never sets failed_tests_json on the record, so the DynamoDB item never carries it and the ClickHouse column stays at its DEFAULT ''.
3. Queried but always empty. torchci/clickhouse_queries/crcr_hud_results/query.sql:15 selects failed_tests_json — reading a column that is always the empty string.
4. Never consumed. Neither CrcrJobRow (torchci/pages/crcr/[org]/[repo].tsx) nor CrcrPrResult (torchci/components/crcr/CrcrPrSection.tsx) declares the field, and the Page 2 query (crcr_backend_dashboard/query.sql) doesn't select it at all.

What does work is the aggregate count — the per-repo dashboard tooltip renders Tests: {passed_tests}/{total_tests} passed from the test-results input. So backends can report how many tests failed, but not which.

Why it matters

The per-backend dashboard is the self-service CI health page L2 repos are given in exchange for onboarding, and drilling into a failure is the main thing a backend maintainer wants from it. Today a red cell is terminal — the only next step is to follow the external workflow-run link into the downstream repo's own CI.

Suggested scope

1. Add a callback-action input for failed-test detail (name/classname/message/stacktrace), consistent with the existing test-results JSON-string convention.
2. Extract and persist it in extractDynamoRecord(), applying the caps from [CRCR] Three of four RFC-0054 payload caps are unimplemented #8548 (they were specified for exactly this field and are a prerequisite, not a follow-up).
3. Add failed_tests_json to crcr_backend_dashboard/query.sql and to CrcrJobRow.
4. Render the drill-down on click, per the Page 2 spec.

Steps 1-2 and 3-4 are separable if it's easier to land the ingest path first.

References

• RFC-0054 — Page 2 spec · Unresolved Question #1 · full doc
• clickhouse_db_schema/default.crcr_workflow_job/schema.sql — failed_tests_json String DEFAULT '' COMMENT 'JSON array of failed/errored test details'