[ty] Fix `ClassLiteral.into_callable` for dataclasses (#19192)
## Summary
Change `ClassLiteral.into_callable` to also look for `__init__` functions
of type `Type::Callable` (such as synthesized `__init__` functions of
dataclasses).
Fixes https://github.com/astral-sh/ty/issues/760
## Test Plan
Add subtype test
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Co-authored-by: David Peter <mail@david-peter.de>
