[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/subscript/bytes.md

@@ -49,5 +49,5 @@ def _(m: int, n: int):
 def _(s: bytes) -> bytes:
     byte_slice2 = s[0:5]
     # TODO: Support overloads... Should be `bytes`
-    reveal_type(byte_slice2)  # revealed: @Todo(return type of decorated function)
+    return reveal_type(byte_slice2)  # revealed: @Todo(return type of decorated function)
 ```