ci: update checkout and setup-python to Node.js 24 compatible versions

[Andreas-Garcia]

Description

Update GitHub Actions workflows to use Node.js 24–compatible versions of actions/checkout and actions/setup-python.

Key changes:

• actions/checkout: v4 → v6 in all workflows
• actions/setup-python: v5 → v6 in all workflows
• Workflows updated: lint-and-test.yml, publish-docs-bundle.yml, publish.yml

CHANGELOG updated under [Unreleased] → CI with an entry for this change.

Related Issues

None

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Test addition/update
• CI/CD or infrastructure change

Pre-PR Checklist

Code Quality

• Removed commented-out code
• No hardcoded credentials, API keys, or secrets
• Ran pre-commit hooks: pre-commit run --all-files

Tests

• All tests pass: pytest
• Coverage meets threshold (≥85%)
• New features have corresponding tests (N/A)
• Bug fixes include regression tests (N/A)

Documentation

• Updated docstrings for new functions/classes (N/A)
• Updated README if adding new features or changing behavior (N/A)
• Updated CONTRIBUTING.md if changing development workflow (N/A)
• Added/updated type hints where appropriate (N/A)
• Updated CHANGELOG.md with changes

Git Hygiene

• Commit messages follow the commit message convention
• No merge conflicts with target branch
• Branch is up to date with target branch
• No accidental commits (large files, secrets, personal configs)

Breaking Changes

• This PR includes breaking changes
• Breaking changes are clearly documented below
• Migration path is provided (if applicable)

Breaking Changes Description

N/A

Testing Instructions

How to Test

1. Push the branch and open a PR; ensure the "Lint and Test" workflow runs.
2. Confirm all jobs (Lint, Test matrix) complete successfully.
3. Optionally trigger the publish workflows (e.g. docs bundle) if you have permissions to verify checkout/setup-python there.

Test Results

CI workflow runs successfully with the new action versions.

Additional Context

• actions/checkout@v6 and actions/setup-python@v6 are compatible with Node.js 24 (GitHub’s default runner environment).
• No behavior change for Python or repo checkout; this is a version bump for the actions only.

Checklist for Reviewers

• Code follows project conventions and style
• Logic is sound and well-structured
• Error handling is appropriate
• CI tests pass on all platforms and Python versions
• Test coverage is adequate for the changes
• Public API changes are documented (N/A)
• Breaking changes are clearly marked and documented (N/A)
• All review comments are addressed
• No unresolved discussions