[ty] Fix implementation of `Top[Callable[..., object]]` (#22145)
## Summary
Add a proper representation for the `Callable` top type, and use it to
get `callable()` narrowing right.
Closes https://github.com/astral-sh/ty/issues/1426.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
