feat: implement Application layer for adding favorites / お気に入り追加のApplication層実装

[zigzagdev]

Motivation / 目的

Following the Infra layer (#561), this PR implements the Application layer for the favorites feature: an AddFavoriteUseCase that lets a user add a world heritage site to their favorites via FavoriteRepositoryInterface.

No UseCommand or Dto is introduced here — handle() only needs user_id and world_heritage_id, and the operation returns void, so passing primitives directly follows the existing DeleteUserUseCase pattern rather than adding an object wrapper for two scalar values.

What I have done / 実施内容

• Added AddFavoriteUseCase::handle(int $userId, int $worldHeritageSiteId): void, delegating to FavoriteRepositoryInterface::addFavorite()
• Added a unit test for AddFavoriteUseCase using Mockery

Test Results / テスト結果

• test_handle_calls_repository_addFavorite_with_given_ids
• test_handle_propagates_exception_when_user_not_found

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.28%. Comparing base (4c6e5a0) to head (8755795).

Additional details and impacted files

@@                   Coverage Diff                    @@
##             feat/user-favorite     #562      +/-   ##
========================================================
+ Coverage                 63.18%   63.28%   +0.09%     
- Complexity                 1635     1640       +5     
========================================================
  Files                       138      140       +2     
  Lines                      8475     8497      +22     
========================================================
+ Hits                       5355     5377      +22     
  Misses                     3120     3120              

Files with missing lines	Coverage Δ
...ndUseCases/UseCase/Favorite/AddFavoriteUseCase.php	100.00% <100.00%> (ø)
...s/Tests/CommandUseCases/AddFavoriteUseCaseTest.php	100.00% <100.00%> (ø)

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[zigzagdev]

Ok