[ty] Fix incorrect diagnostic when calling __setitem__ (#19645)

## Summary

Resolves https://github.com/astral-sh/ty/issues/862 by not emitting a
diagnostic.

## Test Plan

Add test to show we don't emit the diagnostic

crates/ty_python_semantic/resources/mdtest/narrow/assignment.md

@@ -253,7 +253,6 @@ does["not"]["exist"] = 0
 reveal_type(does["not"]["exist"])  # revealed: Unknown
 
 non_subscriptable = 1
-# error: [non-subscriptable]
 non_subscriptable[0] = 0
 # error: [non-subscriptable]
 reveal_type(non_subscriptable[0])  # revealed: Unknown