Migrate prototype tests to pytest

[Fiyiware]

Context

The prototype currently runs its tests via a single script, prototype/tests/test_basic.py, invoked directly with python tests/test_basic.py. As the test surface grows, a real test runner makes failures easier to read and selective runs possible.

What to do

• Convert the existing assertions in prototype/tests/test_basic.py into pytest-style test functions (def test_*).
• Add pytest to prototype/requirements.txt (dev section is fine).
• Ensure pytest discovers and runs the suite from the prototype/ directory.
• Update the CI workflow and CONTRIBUTING.md so the documented command matches (pytest instead of python tests/test_basic.py).

Acceptance criteria

• pytest runs green locally on Python 3.10/3.11/3.12.
• CI still passes on all three versions.
• No change in what is actually being tested — this is a pure migration, not new coverage.

Good to know

Keep the diff small and mechanical. If you spot a missing test case while migrating, open a separate issue/PR for it rather than mixing it in here.