79f8473e51
## Summary Subtyping was already modeled, but assignability also needs an explicit branch. Removes 921 ecosystem false positives. ## Test Plan New Markdown tests.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/red_knot_test/README.md for documentation of this test format.