c5d654bce8
## Summary Extends literal promotion to apply to any generic method, as opposed to only generic class constructors. This PR also improves our literal promotion heuristics to only promote literals in non-covariant position in the return type, and avoid promotion if the literal is present in non-covariant position in any argument type. Resolves https://github.com/astral-sh/ty/issues/1357.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.