bug: harness reviewer is failing on codebuild hosted runners.

### Problem
Example: https://github.com/aws/agentcore-cli/actions/runs/29121238023/job/86456628695?pr=1739

```
Run actions/setup-python@v6
Installed versions
Version 3.12 was not found in the local cache
Error: The version '3.12' with architecture 'x64' was not found for amzn 2023.
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
```

Looks like the new machines don't support the requested version of python.

This is causing failures in CI, introducing noise to review environments.

### Definition of Done
- fix the workflow to use a compatible runner OR downgrade the version to something compatible with the runner.

Start with the workflow run linked in the issue and locate the workflow step using actions/setup-python@v6. Check which CodeBuild-hosted runner and Python version are requested, then make the runner and version compatible and rerun the affected review workflow to confirm CI succeeds without the setup error.

Written by the indexing model from the issue text.