fix(backend): カスタム絵文字の一括インポートをした時にHTTPプロキシの除外設定が効かないのを修正

[samunohito]

What

DownloadServiceによるファイル取得時に、HTTPプロキシの除外設定に従って適切なhttpAgent/httpsAgentを使用するようにしました。

Why

fix #8766

Additional info (optional)

元issueのSteps to Reproduceどおりに操作して問題の発生を確認
↓
修正後のソースで同じ手順で捜査してインポートが動作するのを確認

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

Attention: Patch coverage is 52.38095% with 20 lines in your changes missing coverage. Please review.

Project coverage is 38.65%. Comparing base (ebd06be) to head (af37381).

Files with missing lines	Patch %	Lines
packages/backend/src/core/HttpRequestService.ts	55.00%	16 Missing and 2 partials ⚠️
packages/backend/src/core/DownloadService.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #15431      +/-   ##
===========================================
+ Coverage    37.65%   38.65%   +1.00%     
===========================================
  Files         1607     1607              
  Lines       204854   204887      +33     
  Branches      3717     4018     +301     
===========================================
+ Hits         77135    79205    +2070     
+ Misses      127071   125038    -2033     
+ Partials       648      644       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.