[Preferential] Fix enumerability check in PreferentialGPSampler

[contramundum53]

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This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
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Reference Issues/PRs

What does this implement/fix? Explain your changes.

There was a bug in PreferentialGPSampler that cannot correctly check whether the search space was enumerable. This causes #796. I fix this bug.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (8c48e7e) 69.13% compared to head (fe00b02) 69.13%.
Report is 14 commits behind head on main.

Files	Patch %	Lines
optuna_dashboard/_app.py	30.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #797   +/-   ##
=======================================
  Coverage   69.13%   69.13%           
=======================================
  Files          35       35           
  Lines        2378     2388   +10     
=======================================
+ Hits         1644     1651    +7     
- Misses        734      737    +3     

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[contramundum53]

The CI will fix with optuna/optuna#5229.

[c-bata]

Thank you for the swift fix!