M2-T1: Python test infrastructure — pytest + coverage ≥80%

[hartsock]

Summary

Set up the Python test infrastructure that all subsequent Python tests depend on. Every Python issue in M2–M6 builds on this foundation.

Acceptance Criteria

• pyproject.toml gains [tool.pytest.ini_options] and [tool.coverage.run] sections with --cov-fail-under=80
• python/tests/conftest.py created with a mock_gitxtend fixture (MagicMock for all ~25 public functions in gitxtend._gitxtend)
• Test dependencies declared in pyproject.toml optional group dev: pytest, pytest-cov, pytest-mock
• CI python-e2e job updated: replace raw python python/tests/test_e2e.py with pytest python/tests/ --cov=gitxtend --cov-fail-under=80
• .githooks/pre-push updated to match CI (hook parity rule)
• pytest python/tests/ passes locally after maturin develop

Files

• pyproject.toml — add pytest + coverage config
• python/tests/conftest.py — new: mock_gitxtend fixture
• .github/workflows/ci.yml — update python-e2e job
• .githooks/pre-push — parity update

Notes

The mock_gitxtend fixture patches gitxtend._gitxtend so unit tests never call the compiled extension. The existing test_e2e.py is kept as an integration test (marked --integration) but does not block the coverage gate.

Beaver (MacBook agent, Claude Sonnet 4.6)

[hartsock]

Coverage-gate clarification from the PR #42 review pass: for this M2 infrastructure issue, the current gate intentionally runs all pytest tests under python/tests/ with --cov=gitxtend --cov-fail-under=80. That means the existing integration-marked E2E tests contribute to the coverage result for now; a future split can move integration tests to a separate non-coverage job if we decide unit-only coverage is the better shape.