test(backend): Add test for Passkey API

[yunochi]

What

Add some test for Passkey Sign In API.

Why

#14577 (comment)

Additional info (optional)

I still don't know how to simulate webauthn tokens, so I couldn't create full E2E test.
The part that verifies the token has been replaced with a mock.

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。
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.65%. Comparing base (89841e4) to head (a9e857b).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #14635      +/-   ##
===========================================
+ Coverage    39.57%   39.65%   +0.08%     
===========================================
  Files         1544     1544              
  Lines       193244   193244              
  Branches      3563     2560    -1003     
===========================================
+ Hits         76470    76631     +161     
+ Misses      116209   116048     -161     
  Partials       565      565              

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[syuilo]

🙏🏿