Add openvino backend support for numpy.hstack

[Imokutmfon]

please review @rkazants

Fixes openvinotoolkit/openvino#29361

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.55%. Comparing base (5f2793e) to head (bd8fa37).

Files with missing lines	Patch %	Lines
keras/src/backend/openvino/numpy.py	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21257      +/-   ##
==========================================
- Coverage   82.55%   82.55%   -0.01%     
==========================================
  Files         564      564              
  Lines       54638    54649      +11     
  Branches     8497     8499       +2     
==========================================
+ Hits        45105    45114       +9     
- Misses       7442     7443       +1     
- Partials     2091     2092       +1     

Flag	Coverage Δ
keras	82.36% <81.81%> (-0.01%)	⬇️
keras-jax	63.62% <0.00%> (-0.02%)	⬇️
keras-numpy	58.73% <0.00%> (-0.01%)	⬇️
keras-openvino	32.99% <81.81%> (+0.01%)	⬆️
keras-tensorflow	64.02% <0.00%> (-0.02%)	⬇️
keras-torch	63.67% <0.00%> (-0.02%)	⬇️

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[rkazants]

Test failures, need to fix: https://github.com/keras-team/keras/actions/runs/14883684859/job/41797589114?pr=21257

[Imokutmfon]

Hi @rkazants
please review.
I am not authorized to push too

[fchollet]

LGTM, thank you!