[Metal] Support sorting complex numbers

[Lyxot]

Proposed changes

Followup to #3286.

Add complex64 sort, argsort, partition, and argpartition support to the Metal backend

[Copilot]

Pull request overview

Adds Metal backend support for sorting complex64 values (including NaN-aware ordering) and updates Python tests to exercise complex sorting on Metal, as a follow-up to CUDA complex sorting support.

Changes:

• Instantiate Metal block and multi-block sort/partition/merge kernels for complex64.
• Add complex64_t NaN initialization and isnan(complex64_t) support for comparisons in sort kernels.
• Expand Python sort tests to include complex64 unconditionally (including NaN behavior).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
python/tests/test_ops.py	Enables complex64 coverage for sort and complex NaN sort expectations on Metal
mlx/backend/metal/kernels/sort.metal	Instantiates sort-related kernels for complex64 in the static Metal kernel set
mlx/backend/metal/kernels/sort.h	Adds complex64 init value (NaN) and enables NaN-aware comparator behavior for complex64
mlx/backend/metal/kernels/complex.h	Adds isnan(complex64_t) helper used by sort comparator

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[zcbenz]

Looks good to me!