tests: mypy has no MODULE_PROVIDERS entry, so a visible gate on it is impossible and the guard dies on a bare KeyError

[JarryShaw]

Surfaced by #777's cross-review, which tried to add a visible HAS_MYPY gate to tests/vendor/test_vendor_reg_apptype_generator_unit.py and could not.

mypy has no MODULE_PROVIDERS entry. Confirmed on origin/main (110381b63): grep -n "'mypy'" tests/_dependency_gates.py returns nothing. It ships in Pipfile [dev-packages] and in no pyproject.toml extra, so nothing in the provider table describes it.

Consequence: a skipUnless-shaped gate on mypy cannot be added at all. Measured — adding HAS_MYPY = importlib.util.find_spec('mypy') is not None plus @unittest.skipUnless to that test file alone, touching nothing else, makes python -m unittest tests.test_tier_guard.DependencyGateCoverageTests report Ran 9 tests ... FAILED (failures=2, errors=4). Root cause is a bare KeyError: 'mypy' at tests/_dependency_gates.py:682:

providers = MODULE_PROVIDERS[module.partition('.')[0]]

unconditional, and upstream of any DEPENDENCY_GATE_EXCLUSIONS filtering — so no in-test exclusion rescues it. The same bare subscript appears at :635 and :1315.

Two defects, not one.

1. The missing entry. mypy needs a MODULE_PROVIDERS entry, and probably a DEPENDENCY_GATE_EXCLUSIONS one too, since mypy is a lint-tier tool and gating a test on it should not demand it be added to a pytest install line.
2. The diagnostic. A missing entry surfaces as an opaque KeyError from a table lookup, with nothing naming the fix. _gates_of (:1008-1021) walks only node.decorator_list for skipUnless calls, so the guard's whole purpose is to audit gates — and the first thing it does with an unrecognised one is crash. It should say which module is unknown and that a MODULE_PROVIDERS entry is what it wants. Compare :1239-1241, which already produces exactly that kind of message for a related case.

Why this matters beyond mypy. #777 was pushed into the inline try/except ImportError: self.skipTest(...) form instead, which works but is invisible to the guard by construction — it is not a decorator, so _gates_of never sees it. That is the "dark test" hazard #745 named and #766 records another instance of: the guard can only audit the gate shape that this table makes possible. Every tool in [dev-packages] but no extra has the same hole.

Blocked on #774, which owns tests/_dependency_gates.py (head 1b63e9899, worker live). Checkable blocker: #774 merged.

[JarryShaw]

Unblocked — #774 merged at 12:15:52Z as 35a79016b, so tests/_dependency_gates.py is free. Dispatching.

Note for whoever picks this up: #766 now waits on this issue rather than on #774, because both want _dependency_gates.py and two agents editing it would clobber each other. So the MODULE_PROVIDERS entry for mypy and the bare-KeyError diagnostic land here first; #766's HAS_ISORT gate follows.