ci: update the pinned mise version

[johnnewman-square]

Summary

• Upgrade the pinned mise action from v3.6.3 to v4.3.0.
• Pin the Mise binary to version 2026.9.1 for deterministic CI setup.
• Move the action runtime from deprecated Node 20 to Node 24.

Context

The Listable test workflow for PR #630 failed before compilation because the Mise version endpoint advertised v2026.9.3 while the corresponding GitHub release was not yet published. All three matrix jobs received a 404 during the Set up Mise step.

Pinning the Mise binary avoids relying on the latest-version publication sequence and prevents future CI runs from adopting new Mise releases without an intentional update.

Testing

• Parsed the updated workflow successfully with Ruby YAML.
• Verified the patch with git diff --check.
• Mise 2026.9.1 successfully ran the complete iOS 15.4, 16.2, and 17.2 test matrix on the previous revision of this PR.
• The updated version-pinned workflow will be validated again by GitHub Actions.

Checklist

• No changelog entry is needed because this is an internal CI-only change.