feat: Implement longest_positive_streak function

Implements the `longest_positive_streak` function to find the length of the longest run of consecutive positive numbers in a list.

Also includes:
- A comprehensive pytest test suite in `tests/test_streak.py`.
- A GitHub Actions workflow (`.github/workflows/pytest.yml`) to run tests automatically on push and pull requests.
- A `.gitignore` file to exclude Python cache files.

Author: google-labs-jules[bot]