Implement nanmedian function in keras.ops

[shashaka]

Adds keras.ops.nanmedian, which computes the median of the input tensor along a specified axis while ignoring NaN values.
Supported across NumPy, TensorFlow, PyTorch, and JAX backends. Not supported on OpenVINO.

[gemini-code-assist]

Code Review

This pull request introduces the nanmedian operation across multiple Keras backends (JAX, TensorFlow, PyTorch, and NumPy) and adds the corresponding public API and tests. Feedback focuses on ensuring consistent behavior for edge cases, specifically handling empty axis reductions in the NumPy backend to prevent ValueError and removing a redundant check in the PyTorch implementation. A test assertion also needs to be updated to correctly verify the identity behavior for empty axes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.85%. Comparing base (28a83c5) to head (f9d77b7).

Files with missing lines	Patch %	Lines
keras/src/backend/jax/numpy.py	33.33%	2 Missing ⚠️
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   #22585      +/-   ##
==========================================
- Coverage   83.28%   79.85%   -3.44%     
==========================================
  Files         596      596              
  Lines       68089    68125      +36     
  Branches    10607    10610       +3     
==========================================
- Hits        56711    54401    -2310     
- Misses       8634    11020    +2386     
+ Partials     2744     2704      -40     

Flag	Coverage Δ
keras	79.68% <88.88%> (-3.41%)	⬇️
keras-jax	24.78% <30.55%> (-34.89%)	⬇️
keras-numpy	55.35% <63.88%> (+<0.01%)	⬆️
keras-openvino	53.33% <50.00%> (-0.01%)	⬇️
keras-tensorflow	61.04% <69.44%> (+<0.01%)	⬆️
keras-torch	59.86% <69.44%> (+<0.01%)	⬆️

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[hertschuh]

Thanks for adding this!

Can you also rebase?

[hertschuh]

Thanks!