Implement cbrt function in keras.ops

[shashaka]

Adds keras.ops.cbrt, which computes the real-valued cube root of each element in the input tensor.
Supported across NumPy, TensorFlow, PyTorch, and JAX backends. Not supported on OpenVINO.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.75%. Comparing base (18ab462) to head (c2e30f8).

Files with missing lines	Patch %	Lines
keras/api/_tf_keras/keras/ops/__init__.py	0.00%	1 Missing ⚠️
keras/api/_tf_keras/keras/ops/numpy/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21453   +/-   ##
=======================================
  Coverage   82.74%   82.75%           
=======================================
  Files         565      565           
  Lines       55300    55340   +40     
  Branches     8624     8631    +7     
=======================================
+ Hits        45756    45794   +38     
- Misses       7439     7441    +2     
  Partials     2105     2105           

Flag	Coverage Δ
keras	82.56% <95.00%> (+<0.01%)	⬆️
keras-jax	63.32% <37.50%> (-0.02%)	⬇️
keras-numpy	58.53% <52.50%> (-0.01%)	⬇️
keras-openvino	33.77% <22.50%> (-0.01%)	⬇️
keras-tensorflow	63.75% <50.00%> (-0.01%)	⬇️
keras-torch	63.36% <50.00%> (-0.01%)	⬇️

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