Add GitHub Actions test workflow

[adulau]

Motivation

• Provide CI that runs the test suite on push and pull_request to catch regressions automatically.

Description

• Add .github/workflows/tests.yml which runs python -m pytest on ubuntu-latest across Python versions 3.10, 3.11, 3.12, and 3.13, using actions/checkout@v4, actions/setup-python@v5 with pip caching, and installing test deps via python -m pip install -e . pytest.

Testing

• Ran python -m pytest -q locally and received 31 passed indicating the test suite succeeds.

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Codex Task