Add pytest test suite and CI test runner

[midaa1]

Summary

• Added a pytest test suite with 47 tests covering unit, integration, and API layers
• Updated the CI pipeline to install dependencies, set up Python 3.11, and run pytest -v on every PR

What's tested

File	What it covers
test_metrics.py	Precision, accuracy, and Euclidean distance metric functions
test_gaze_tracker.py	squash, normalizeData, model registry, trian_and_predict, predict,
predict_new_data_simple
test_session_model.py	Session model creation, to_dict serialization
test_routes.py	/health, /calib_validation, /batch_predict endpoints (success, error, method rejection)
test_config.py	Hyperparameter grid structure and values
test_nan_conversion.py	NaN/Inf → None conversion for JSON serialization (floats, numpy types, nested structures)

Changes

• tests/ — new test suite (conftest + 6 test modules)
• pytest.ini — pytest configuration
• .github/workflows/main.yml — CI now runs pytest -v instead of checkout-only