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8d5d41cc - [ty] Improve type context support for `__setitem__` dunder calls (#23800)

[ty] Improve type context support for `__setitem__` dunder calls (#23800) Resolves https://github.com/astral-sh/ty/issues/2986. I also ended up refactoring some things here, see https://github.com/astral-sh/ty/issues/3001 for details.