[red-knot] detect invalid return type (#16540)

## Summary

This PR closes #16248.

If the return type of the function isn't assignable to the one
specified, an `invalid-return-type` error occurs.
I thought it would be better to report this as a different kind of error
than the `invalid-assignment` error, so I defined this as a new error.

## Test Plan

All type inconsistencies in the test cases have been replaced with
appropriate ones.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/annotations/deferred.md

@@ -38,7 +38,8 @@ If `__future__.annotations` is imported, annotations *are* deferred.
 ```py
 from __future__ import annotations
 
-def get_foo() -> Foo: ...
+def get_foo() -> Foo:
+    return Foo()
 
 class Foo: ...