No automated tests for the Python layer (bin/python/)

[corgab]

Problem

The PHP side is well covered (Pest, ~90% line coverage), but the Python scripts in bin/python/ have no automated tests and no test harness. This is now a concrete gap because circuit.py contains non-trivial pure logic:

• _validate_gates() — qubit-index range validation, measure targets handling, the isinstance(index, bool) guard, and the _QUBIT_INDEX_KEYS allowlist that must stay in sync with _build_circuit().

These are exactly the kind of guards that get silently broken by a future refactor with nothing to catch the regression. Today they are only verified by ad-hoc manual invocation.

Proposal

• Add a minimal pytest harness under bin/python/ (e.g. tests/ + requirements-dev.txt with pytest).
• _validate_gates() and _build_circuit's gate-dispatch are importable without the Braket SDK (braket is imported lazily inside the functions), so the validation logic can be unit-tested with no AWS/Braket dependency.
• Cover: valid circuits pass; out-of-range / negative / non-int indices raise ValueError; scalar measure.targets; measure-all (null); unknown gate type.
• Wire pytest into the CI workflow (.github/workflows/tests.yml) as a separate step.

Acceptance

• pytest runs in CI and covers _validate_gates (and ideally _build_circuit with a mocked/stubbed Braket Circuit).
• The bool-is-int guard and the _QUBIT_INDEX_KEYS/_build_circuit coupling have explicit regression tests.