[red-knot] fix inheritance-cycle detection for generic classes (#17620)
## Summary
The `ClassLiteralType::inheritance_cycle` method is intended to detect
inheritance cycles that would result in cyclic MROs, emit a diagnostic,
and skip actually trying to create the cyclic MRO, falling back to an
"error" MRO instead with just `Unknown` and `object`.
This method didn't work properly for generic classes. It used
`fully_static_explicit_bases`, which filter-maps `explicit_bases` over
`Type::into_class_type`, which returns `None` for an unspecialized
generic class literal. So in a case like `class C[T](C): ...`, because
the explicit base is an unspecialized generic, we just skipped it, and
failed to detect the class as cyclically defined.
Instead, iterate directly over all `explicit_bases`, and explicitly
handle both the specialized (`GenericAlias`) and unspecialized
(`ClassLiteral`) cases, so that we check all bases and correctly detect
cyclic inheritance.
## Test Plan
Added mdtests.