Enhance delete artifacts APIs and add tests for them

[y0z]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

None

What does this implement/fix? Explain your changes.

• Add tests for delete study/trial artifacts APIs
• Make delete study/trial artifacts APIs to be able to properly handle the ArtifactNotFound exception, i.e., returning 404 rather than causing internal server error

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (616f08c) 68.20% compared to head (99a54cb) 68.81%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
+ Coverage   68.20%   68.81%   +0.60%     
==========================================
  Files          35       35              
  Lines        2334     2344      +10     
==========================================
+ Hits         1592     1613      +21     
+ Misses        742      731      -11     

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[contramundum53]

LGTM!

[c-bata]

LGTM! Thank you for your contribution.