`experimental-inspect`: Add `stubtest` test in CI to ensure generated stubfiles match binary interfaces

There is no test asserting stubtest is happy, only a commented-out TODO at pytests/noxfile.py:49.

There are some issues though that would have to be fixed before simply enabling a stubtest test in CI:

1. pyo3_pytests isn't importable in the stub layout. type_checker does copytree("stubs", "pyo3_pytests"), making it a mixed-project source dir of only .pyi files, so maturin generates no __init__.py, #[pymodule_init] never runs and no submodule is importable. Fine for the static-only mypy session, fatal for stubtest.

2. 43 unrelated gaps remain once it is importable:

   Count	Gap
   16	__all__ is set at runtime by #[pymodule] but never emitted into the stubs
   15	submodules aren't declared in __init__.pyi; stubtest doesn't accept the incomplete-module __getattr__ as covering them
   6	@disjoint_base isn't emitted for classes that can't be subclassed
   3	annotations, consts, exception aren't registered in sys.modules by pytests/src/lib.rs (fixture gap)
   1	Number.__pow__'s modulo has a runtime default the stub doesn't express
   1	the inner pyo3_pytests.pyo3_pytests module has no .pyi

Two smaller adjacent defects, also left alone: slot parameters with a runtime default are emitted without one (__pow__'s modulo, __get__'s owner), and reflected operators (__radd__, __rpow__, …) exist at runtime but never reach the stubs.

Start at the commented-out TODO in pytests/noxfile.py:49 and inspect the type_checker session's stubs-to-pyo3_pytests layout. Then read pytests/src/lib.rs and compare the generated stubs with the listed stubtest gaps, including submodules, all, disjoint bases, defaults, and reflected operators. Done means CI runs stubtest successfully without the documented gaps.