Improve `flatiter` indexing by using ndarray index preparation

The `flatiter`` object now shares the same index preparation logic as

`ndarray`, ensuring consistent behavior and fixing several issues where

invalid indices were previously accepted or misinterpreted.

Key fixes and improvements:

* Stricter index validation

``arr.flat[[True, True]]`` and ``arr.flat[[1.0, 1.0]]`` were incorrectly

treated as ``arr.flat[[1, 1]]``. They now raise an `IndexError`` (unless

``arr.flat[[True, Truee]]`` is a valid boolean index)

* Fixed crash when assigning to an empty index tuple:

``arr.flat[()] = 0`` previously crashed the Python interpreter. It now

correctly assigns to the entire array.

* Consistent error types:

Certain invalid `flatiter` indices that previously raised `ValueError`

now correctly raise `IndexError`, aligning with `ndarray` behavior.

* Improved error messages:

The error message for unsupported index operations now provides more

specific details, including explicitly listing the valid index types,

instead of the generic ``IndexError: unsupported index operation``.