Implement kaiser function in keras.ops

[shashaka]

Adds keras.ops.kaiser, which computes the Kaiser window. Supported across NumPy, TensorFlow, PyTorch, and JAX backends, but not supported on OpenVINO.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.58%. Comparing base (dfaca0e) to head (180fd2d).

Files with missing lines	Patch %	Lines
keras/src/ops/numpy.py	84.61%	1 Missing and 1 partial ⚠️
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   #21303   +/-   ##
=======================================
  Coverage   82.57%   82.58%           
=======================================
  Files         564      564           
  Lines       54677    54707   +30     
  Branches     8500     8501    +1     
=======================================
+ Hits        45152    45178   +26     
- Misses       7435     7438    +3     
- Partials     2090     2091    +1     

Flag	Coverage Δ
keras	82.39% <86.66%> (+<0.01%)	⬆️
keras-jax	63.61% <60.00%> (-0.01%)	⬇️
keras-numpy	58.74% <63.33%> (+<0.01%)	⬆️
keras-openvino	33.04% <36.66%> (+<0.01%)	⬆️
keras-tensorflow	64.02% <60.00%> (-0.01%)	⬇️
keras-torch	63.68% <60.00%> (-0.01%)	⬇️

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[fchollet]

Nice work, thank you! LGTM