ci: Upgrade GitHub Actions for Node 24 compatibility

Merge https://github.com/google/adk-python/pull/4426

## Summary

Upgrade GitHub Actions to their latest versions to ensure compatibility with Node 24, as Node 20 will reach end-of-life in April 2026.

## Changes

| Action | Old Version(s) | New Version | Release | Files |
|--------|---------------|-------------|---------|-------|
| `actions/checkout` | [`v4`](https://github.com/actions/checkout/releases/tag/v4) | [`v6`](https://github.com/actions/checkout/releases/tag/v6) | [Release](https://github.com/actions/checkout/releases/tag/v6) | mypy-new-errors.yml, mypy.yml |
| `actions/setup-python` | [`v5`](https://github.com/actions/setup-python/releases/tag/v5) | [`v6`](https://github.com/actions/setup-python/releases/tag/v6) | [Release](https://github.com/actions/setup-python/releases/tag/v6) | mypy-new-errors.yml, mypy.yml |

## Context

Per [GitHub's announcement](https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/), Node 20 is being deprecated and runners will begin using Node 24 by default starting March 4th, 2026.

### Why this matters

- **Node 20 EOL**: April 2026
- **Node 24 default**: March 4th, 2026
- **Action**: Update to latest action versions that support Node 24

### Security Note

Actions that were previously pinned to commit SHAs remain pinned to SHAs (updated to the latest release SHA) to maintain the security benefits of immutable references.

### Testing

These changes only affect CI/CD workflow configurations and should not impact application functionality. The workflows should be tested by running them on a branch before merging.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4426 from salmanmkc:upgrade-github-actions-node24 a574f92c72502ae60531163da60f7820e01380f3
PiperOrigin-RevId: 868281419