Switching test models to Keras3 format

[jezsadler]

Also switching tensorflow to tensorflow-cpu.

Currently test_scaling_NN_block is failing, haven't figured out why. Will investigate further...

Also, notebook tests are failing due to sympy update.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.55%. Comparing base (024f526) to head (5cd55d6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #146    +/-   ##
========================================
  Coverage   92.55%   92.55%            
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  Files          32       32            
  Lines        1948     1948            
  Branches      373      475   +102     
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  Hits         1803     1803            
  Misses         74       74            
  Partials       71       71            

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