test(corekit): the sentinel-convention pin reads literals the #1946 links replaced #1957

tests/corekit/test_sentinel_exports_unit.py::SentinelPopulationTests::test_conventions_doc_lists_every_sentinel_in_the_tree fails on main at c8c34a86f. Locally it gives 4 failed with SUBFAILED(sentinel='NO_DEFAULT') and the other sentinels. The first CI leg to report it is #1956's Python 3.9 (KTLO) leg, where it shows as a warning. Main's own Unit Tests runs are still queued, so they will go red once they run.

Cause: #1946 (7f538225b) turned the type column of docs/source/contributing/conventions/sentinel-convention.rst from literals (``NullType``) into roles (:class:`~pcapkit.corekit.sentinels.NullType`). The test still looks for the literal form. #1946's review ran tests/project and the doc guards but not tests/corekit, so the pin there was missed.

Fix: make the test read the type column as either a literal or a :class: role naming the type in pcapkit.corekit.sentinels, and keep both directions of the row check. Before calling it done, also run the other docs-reading tests outside tests/project against main, in case #1946, #1952 or #1953 broke another text pin.