Add workflow to rerun potentially transient failures

[henrymercer]

Workflow runs that are on main, on a release branch, and that were triggered as part of a merge group have already passed CI before being merged. Therefore if they fail, we should make sure that there wasn't a transient failure by rerunning the failed jobs once before investigating further.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• None - I am not validating these changes.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

I'll monitor runs of this workflow once this is merged.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull request overview

Adds a new “Deflake” GitHub Actions workflow to automatically rerun failed jobs once for certain post-merge workflow runs, aiming to filter out transient CI failures on trusted branches.

Changes:

• Introduces .github/workflows/deflake.yml triggered via workflow_run for a curated set of existing workflows.
• Automatically reruns failed jobs on first failure attempt and posts a brief step summary.

Show a summary per file

File	Description
.github/workflows/deflake.yml	New workflow_run-triggered workflow that reruns failed jobs once for selected workflows under specific conditions.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 2