[ty] Don't panic with argument that doesn't actually implement Iterable (#19602)
This eliminates the panic reported in
https://github.com/astral-sh/ty/issues/909, though it doesn't address
the underlying cause, which is that we aren't yet checking the types of
the fields of a protocol when checking whether a class implements the
protocol. And in particular, if a class explictly opts out of iteration
via
```py
class NotIterable:
__iter__ = None
```
we currently treat that as "having an `__iter__`" member, and therefore
implementing `Iterable`.
Note that the assumption that was in the comment before is still
correct: call binding will have already checked that the argument
satisfies `Iterable`, and so it shouldn't be an error to iterate over
said argument. But arguably, the new logic in this PR is a better way to
discharge that assumption — instead of panicking if we happen to be
wrong, fall back on an unknown iteration result.
ruff
130d4e11 - [ty] Don't panic with argument that doesn't actually implement Iterable (#19602)
