Add unit tests for save_trial_user_attrs

[toshihikoyanase]

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This pull request may therefore be ported to Goptuna.
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Reference Issues/PRs

save_trial_user_attrs was introduced to optuna/_app.py in #431. However, it lacks unit tests.

What does this implement/fix? Explain your changes.

• Add test cases for save_trial_user_attrs to python_tests/test_api.py
  • Verify that user_attrs are saved correctly
  • Ensure it can handle an empty dictionary as invalid input

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6d0fef3) 64.00% compared to head (c1c6adc) 64.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #702      +/-   ##
==========================================
+ Coverage   64.00%   64.35%   +0.35%     
==========================================
  Files          35       35              
  Lines        2250     2250              
==========================================
+ Hits         1440     1448       +8     
+ Misses        810      802       -8     

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[c-bata]

LGTM 👍 I will merge this PR after passing CI.