[BUG] `__setitem__` with a single Ellipsis fails

Describe the bug

__setitem__ with a single ellipsis fails, while manually wrapping the ellipsis into a tuple makes it work.

To Reproduce

Include code snippet

>>> a = mx.arange(3)
>>> a[...] = 42
Traceback (most recent call last):
  File "<python-input-29>", line 1, in <module>
    a[...] = 42
    ~^^^^^
ValueError: Cannot index mlx array using the given type.
>>> a[(...,)] = 42
>>> a
array([42, 42, 42], dtype=int32)

Expected behavior

Expected a[...] to be equivalent to a[(Ellipsis,)], similar to indexing with a single integer or a slice: a[key] is a syntax sugar for a[(key,)] for key being an integer, a slice or the Ellipsis singleton.

Desktop (please complete the following information):

• OS Version: [e.g. MacOS 14.1.2]
• Version [e.g. 0.7.0]

$ sw_vers -productVersion 
26.4.1
$ python -c'import mlx.core as mx; print(mx.__version__)'
0.32.1

Additional context
Add any other context about the problem here.

Start by reproducing the reported a[...] and a[(...,)] cases, then trace the array setitem path for a bare Ellipsis versus a tuple-wrapped key. Add a regression test showing both forms assign the same values, and confirm the test passes without changing the existing tuple behavior.