chore(backend): FileServerServiceのunit-testを追加

[samunohito]

What

FileServerServiceのテストを追加しました（現実装を正とし、カバレッジを確保する形で対応）。
FileServerServiceを直接呼び出すわけでは無く、Fastify経由で呼び出すようにしています。Fastify/HTTPにまつわる処理が多いための措置です。

Why

動作確認をしやすくするため

Additional info (optional)

ローカルでpassを確認

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[github-actions]

このPRによるapi.jsonの差分
差分はありません。
Get diff files from Workflow Page

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.58%. Comparing base (cdb958c) to head (bc16748).
⚠️ Report is 25 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #17086       +/-   ##
============================================
+ Coverage    14.03%   63.58%   +49.55%     
============================================
  Files          238     1156      +918     
  Lines        11296   115429   +104133     
  Branches      3736     8147     +4411     
============================================
+ Hits          1585    73397    +71812     
- Misses        7586    39856    +32270     
- Partials      2125     2176       +51     

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[github-actions]

Backend Memory Usage Comparison

Metric	base	head	Diff
RSS	314.35 MB	341.48 MB	27.12 MB (8.62%)

⚠️ Warning: Memory usage has increased by more than 5%. Please verify this is not an unintended change.

See workflow logs for details

[syuilo]

👍