test: `[optional]`-dependency tests have no skip guards — a recurring false "pre-existing failure on main"

[Jammy2211]

The finding

test_autofit/non_linear/search/nest/test_nautilus.py::test__single_core_builds_no_pool
runs a real search.fit, which reaches from nautilus import Sampler
(autofit/non_linear/search/nest/nautilus/search.py:238). nautilus-sampler==1.0.5
ships only in the [optional] extra (pyproject.toml:85), and the test carries
no skip guard.

CI installs [optional] (PyAutoHeart's lib-tests.yml → pip install "./$r[optional]"),
so CI never sees it. Every env installed without those extras — cloud sandboxes, local
venvs — gets a hard ModuleNotFoundError.

It is the only optional-sampler test in the suite without a guard. Precedent exists
throughout: test_blackjax_nuts.py:14, nest/nss/test_search.py:20,
test_arithmetic_jax_trace.py:6, test_fork_context.py:31.

astropy produces the same class of noise: two modules fail collection on its
top-level import, and nine aggregator tests error on it at runtime
(autofit/aggregator/file_output.py:101, summary/aggregate_fits.py:105,165).

Why it matters

This one test has been reported as "pre-existing on clean main" in at least six
PyAutoMind completion records since 2026-08-16, each costing a control run to
re-disprove. complete/2026/08/prior-support-clipper.md:218 already issued the
correction ("read it as environment-specific"), but the looser phrasing propagated
into later PR bodies (#1479, #1480) and records anyway.

Main is not red. Latest main CI
(run 32546158666):
2024 passed, 3 skipped, 0 failed on 3.12, 3.13 and the no-jax leg.

Reproduced on untouched main @ a639226 (py3.12)

env	result
no nautilus-sampler	1 failed, 3 passed — ModuleNotFoundError at search.py:242
pip install nautilus-sampler==1.0.5, same tree	4 passed
full suite, no optional extras	2 collection errors → then 4 failed / 9 errors, all missing-astropy

The fix

Skip-if-missing guards in the existing house pattern — an
importlib.util.find_spec skipif marker, or pytest.importorskip where the import
is module-level. Inert wherever the extras are installed, so no CI coverage changes.

Prior art: this was filed as follow-up 4 of
PyAutoMind/complete/2026/08/uniform-prior-bounds-numpy-path.md ("hygiene candidate:
skip-if-missing markers") but was never turned into a prompt, so nothing acted on it.