Add GitHub Actions workflow to run pytest on pull requests

[AlexanderOnischenko]

Motivation

• Ensure pull requests run the same pytest invocation as the pre-commit hook by adding a CI workflow that executes tests with the not slow and not known_bad markers.

Description

• Add .github/workflows/pytest.yml which triggers on pull_request, sets up Python 3.11, installs dev dependencies with python -m pip install -e ".[dev]", sets PYTHONPATH to .:src, and runs python -m pytest -q -m "not slow and not known_bad".

Testing

• No automated tests were run as part of this change; the new workflow will run python -m pytest -q -m "not slow and not known_bad" automatically on future pull requests.

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Codex Task