Pin Python 3.13.15 for older CLI versions in the multi-language check

[henrymercer]

The "Multi-language repository" PR check is failing intermittently on ubuntu-latest for older CLI versions, for example in this run on main, with ImportError: cannot import name '_ERR_MSG' from 'importlib._bootstrap'.

The check installs Python 3.13 for these CLI versions because they don't work with Python 3.14 (#3212). Version 20261005.85 of the ubuntu-24.04 runner image has Python 3.13.16 in its toolcache, and 3.13.16 includes python/cpython@b0a181d8fc, a backport of the change that removed importlib._bootstrap._ERR_MSG in 3.14. Jobs that still run on the previous image get 3.13.15 and pass.

This PR pins the version to 3.13.15, which setup-python downloads on images that don't have it in their toolcache. The step's condition is unchanged, so newer CLI versions such as default also get 3.13.15, even though they don't need it.

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?

• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

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?

• Other - The "Multi-language repository" PR check will keep failing for older CLI versions.

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]

Warning

• Copilot's review of this pull request may be incomplete because some of the changed files are excluded by your Copilot content exclusion settings. See Excluding content from Copilot for details.

🟢 Approval recommended

The focused version pin addresses the documented CI failure without changing the existing execution condition.

0 open findings

What changed in this PR

Pins Python 3.13.15 in the multi-language PR check to avoid extractor failures with older CodeQL CLI versions.

Changes:

• Pins setup-python to Python 3.13.15.
• Documents the Python 3.13.16 incompatibility.

File	Description
pr-checks/​checks/​multi-language-autodetect.yml	Updates the workflow template’s Python version and rationale.
.github/​workflows/​__multi-language-autodetect.yml	Generated counterpart; excluded from direct review.

Files excluded by content exclusion policy (1)

• .github/workflows/__multi-language-autodetect.yml

🧠 Review effort: Balanced

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