Purpose
-------
OI-511 regression tests — ``oct test`` must surface an actionable install
hint when pytest is missing. Because ``pytest`` is declared only in the
``[project.optional-dependencies].test`` extra, users who install the
runtime-only package and try to run ``oct test`` should see the hint
rather than a bare ``No module named pytest`` traceback.
Responsibilities
----------------
- ``_pytest_missing`` flags the sentinel string pytest emits when not
importable and returns False otherwise.
- ``_PYTEST_INSTALL_HINT`` references the ``oct[test]`` extra so users
discover the canonical install path.
Diagnostics
-----------
Domain: CLI-TESTS
Levels:
L2 — test lifecycle
L3 — assertion details
L4 — deep tracing
Contracts
---------
- Must never claim pytest is missing on a benign pytest failure (e.g.
assertion mismatch).