## Summary
Teach red-knot that `type[...]` is always disjoint from `None` and from
`LiteralString`. Fixes#14925.
This should properly be generalized to "all instances of final types
which are not subclasses of `type`", but until we support finality,
hardcoding `None` (which is known to be final) allows us to fix the
subtype transitivity property test.
## Test Plan
Existing tests pass, added new unit tests for `is_disjoint_from` and
`is_subtype_of`.
`QUICKCHECK_TESTS=100000 cargo test -p red_knot_python_semantic --
--ignored types::property_tests::stable` fails only the "assignability
is reflexive" test, which is known to fail on `main` (#14899).
The same command, with `property_tests.rs` edited to prevent generating
intersection tests (the cause of #14899), passes all quickcheck tests.
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