feat: implement Infra layer for removing favorites / お気に入り解除のInfra層実装

[zigzagdev]

Motivation / 目的

Add the Infra-layer piece needed to remove a favorite: a way to detach a world heritage site from a user's favorites. This is a shared dependency for both the favorites list screen (removing a card) and, later, the toggle heart button on the top/search/detail pages.

お気に入り解除に必要なInfra層の実装です。お気に入り一覧画面でのカード削除と、将来のトグルハートボタン（トップ/検索/詳細ページ）の両方から使われる共通の土台になります。

What I have done / 実施内容

• Added FavoriteRepositoryInterface::removeFavorite(int $userId, int $worldHeritageSiteId): void
• Implemented it in FavoriteRepository using $user->favorites()->detach()
• Throws the same Exception('User not found.') as addFavorite when the user does not exist
• Detaching a heritage that isn't currently favorited is a no-op (no exception)

Test Results / テスト結果

• test_removeFavorite_detaches_world_heritage_site_from_user
• test_removeFavorite_does_not_throw_when_not_favorited
• test_removeFavorite_throws_exception_when_user_not_found

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.38%. Comparing base (298153d) to head (63bdf49).

Additional details and impacted files

@@                   Coverage Diff                    @@
##             feat/user-favorite     #572      +/-   ##
========================================================
+ Coverage                 64.28%   64.38%   +0.09%     
- Complexity                 1677     1682       +5     
========================================================
  Files                       145      145              
  Lines                      8753     8776      +23     
========================================================
+ Hits                       5627     5650      +23     
  Misses                     3126     3126              

Files with missing lines	Coverage Δ
...p/Packages/Domains/Favorite/FavoriteRepository.php	100.00% <100.00%> (ø)
.../Domains/Favorite/Tests/FavoriteRepositoryTest.php	100.00% <100.00%> (ø)

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[zigzagdev]

Ok