mlx - test updates and signbit implementation

[acsweet]

This addresses #19571

Include the following:

• more tests updated as needed by mlx
• signbit implementation
• align left_shift, right_shift, and top_k to match other backends
  • top_k default to sorted adds a bit of overhead, could set this to False and adjust tests
• convert_to_numpy more correct handling of bfloat16
• added placeholders for some unimplemented ops

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.68966% with 46 lines in your changes missing coverage. Please review.

Please upload report for BASE (mlx@65f75bb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
keras/src/backend/mlx/numpy.py	5.12%	37 Missing ⚠️
keras/src/backend/mlx/math.py	0.00%	5 Missing ⚠️
keras/src/backend/mlx/core.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             mlx   #21180   +/-   ##
======================================
  Coverage       ?   76.03%           
======================================
  Files          ?      577           
  Lines          ?    57762           
  Branches       ?     8960           
======================================
  Hits           ?    43922           
  Misses         ?    11806           
  Partials       ?     2034           

Flag	Coverage Δ
keras	75.86% <20.68%> (?)
keras-jax	60.90% <20.68%> (?)
keras-numpy	56.29% <20.68%> (?)
keras-tensorflow	61.19% <20.68%> (?)
keras-torch	60.15% <10.34%> (?)

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[fchollet]

LGTM, thank you!