fix: raise ValueError in deserialize_keras_object when required Keras config keys are missing

[AlgorithmicDeer]

Fixes #22704

Description

deserialize_keras_object silently returned a raw dict when the input was missing class_name but contained Keras-specific keys like module or registered_name. This made it impossible for callers to detect a failed deserialization.
Root cause: The block meant to handle plain Python dicts also accidentally caught malformed Keras configs.
Fix: Added a guard — if the incomplete dict contains module or registered_name (keys that only appear in Keras-serialized objects), raise a descriptive ValueError listing exactly which keys are missing. Plain dicts without those keys continue to pass through unchanged, preserving existing behavior. This is consistent with the same validation already present in the module_objects code path.
Tests added (serialization_lib_test.py):

• test_deserialize_missing_class_name_raises — reproduces the exact bug
• test_deserialize_missing_class_name_with_registered_name_raises — same bug via registered_name
• test_deserialize_plain_dict_no_keras_keys_passthrough — regression guard for plain dicts

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[gemini-code-assist]

Code Review

This pull request enhances the deserialize_keras_object function to explicitly validate Keras configuration dictionaries. It now raises a ValueError when a dictionary contains Keras-specific keys like module or registered_name but lacks the required class_name or config keys, preventing silent failures or incorrect passthrough. Corresponding unit tests were added to verify this behavior and ensure plain dictionaries still pass through correctly. Feedback was provided to simplify the error-raising logic and refine the error message for better readability and alignment with Keras API guidelines.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.96%. Comparing base (e94cb07) to head (34fc240).
⚠️ Report is 37 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #22710   +/-   ##
=======================================
  Coverage   82.95%   82.96%           
=======================================
  Files         596      596           
  Lines       69252    69255    +3     
  Branches    10814    10815    +1     
=======================================
+ Hits        57451    57454    +3     
  Misses       8973     8973           
  Partials     2828     2828           

Flag	Coverage Δ
keras	82.77% <100.00%> (+<0.01%)	⬆️
keras-jax	58.72% <100.00%> (+<0.01%)	⬆️
keras-numpy	54.56% <100.00%> (+<0.01%)	⬆️
keras-openvino	59.43% <100.00%> (+<0.01%)	⬆️
keras-tensorflow	60.28% <100.00%> (+<0.01%)	⬆️
keras-torch	59.06% <100.00%> (+<0.01%)	⬆️

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[hertschuh]

Closing since #22704 was closed as working as designed.