[CI] ubuntu-latest jobs canceled before runner assignment

## Failure signature

GitHub Actions jobs targeting `ubuntu-latest` remain unassigned for approximately 75-86 minutes and then end as canceled. For every occurrence, the job metadata reports `runner_id: 0`, an empty runner name, and zero executed steps.

**Stable signature:** `github-actions-ubuntu-latest-unassigned-runner-canceled`
**Classification:** CI infrastructure failure
**Recurrence:** 2 distinct PRs, 4 confirmed occurrences
**First observed:** 2026-08-06 17:31:57 UTC
**Latest observed:** 2026-08-06 19:06:12 UTC

## Confirmed occurrences

| PR / head | Workflow / job | Run / job | Observed interval |
| --- | --- | --- | --- |
| #26418 / `8c12572c897e1c6e25cb94f8e93fbed1c6fd074c` | Autoformat code v2 / Autoformat and push | [run 31123395950](https://github.com/dotnet/macios/actions/runs/31123395950), [job 92688718765](https://github.com/dotnet/macios/actions/runs/31123395950/job/92688718765) | 17:31:57-18:47:47 UTC |
| #26418 / `8c12572c897e1c6e25cb94f8e93fbed1c6fd074c` | PR yamllint check / yamllint check | [run 31123395914](https://github.com/dotnet/macios/actions/runs/31123395914), [job 92688718660](https://github.com/dotnet/macios/actions/runs/31123395914/job/92688718660) | 17:31:57-18:50:38 UTC |
| #26418 / `8c12572c897e1c6e25cb94f8e93fbed1c6fd074c` | zizmor / zizmor | [run 31123396010](https://github.com/dotnet/macios/actions/runs/31123396010), [job 92688718832](https://github.com/dotnet/macios/actions/runs/31123396010/job/92688718832) | 17:31:57-18:57:50 UTC |
| #26408 / `cde4ac8065b5ff26886d9575036ff76bfbc8eb3c` | zizmor / zizmor | [run 31124297839](https://github.com/dotnet/macios/actions/runs/31124297839), [job 92691589511](https://github.com/dotnet/macios/actions/runs/31124297839/job/92691589511) | 17:50:35-19:06:12 UTC |

## Impact

The affected PR checks remain canceled/failing even though no workflow step or repository code executed. This can block otherwise healthy PRs from becoming merge-ready.

## Investigation

- All four jobs target `ubuntu-latest`.
- All report `runner_id: 0`, an empty runner name, and no steps.
- Autoformat, yamllint, and zizmor contain no workflow concurrency rule that explains these cancellations.
- The cancellation initiator is not exposed in retained job metadata.
- Azure DevOps validation for the affected PRs continued without the same symptom.
- Separate searches across open and closed `dotnet/macios` issues for the stable signature, no-runner cancellations, workflow names, and `ubuntu-latest` found no matching issue.

## Retry and workaround

No retry was submitted by the CI monitor: its retry policy authorizes Azure DevOps retries, not GitHub Actions reruns. There is no confirmed repository-side workaround; recovery requires GitHub-hosted runner capacity or a repository owner rerunning the affected workflow after capacity recovers.

GitHub run and job metadata are retention-limited. No step logs exist because the jobs never received a runner.

Start with the linked GitHub Actions runs and job metadata for PRs #26418 and #26408, confirming the runner_id, runner name, assignment delay, and absence of steps. Compare the Autoformat code v2, PR yamllint check, and zizmor workflows for repository-side causes; done means the cancellation cause is established and a validated recovery or repository-side mitigation is documented.

Written by the indexing model from the issue text.