Implement blackman function in keras.ops

[shashaka]

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.

Project coverage is 82.60%. Comparing base (b2fe0a7) to head (e6798c7).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
keras/src/ops/numpy.py	80.00%	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   #21235   +/-   ##
=======================================
  Coverage   82.60%   82.60%           
=======================================
  Files         564      564           
  Lines       54498    54535   +37     
  Branches     8469     8470    +1     
=======================================
+ Hits        45017    45050   +33     
- Misses       7397     7400    +3     
- Partials     2084     2085    +1     

Flag	Coverage Δ
keras	82.41% <88.23%> (+<0.01%)	⬆️
keras-jax	63.67% <44.11%> (-0.02%)	⬇️
keras-numpy	58.80% <47.05%> (-0.02%)	⬇️
keras-openvino	32.99% <29.41%> (+<0.01%)	⬆️
keras-tensorflow	64.09% <64.70%> (-0.01%)	⬇️
keras-torch	63.74% <44.11%> (-0.02%)	⬇️

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[fchollet]

Thanks for the PR!