test: add unit tests for factories and UserEntity to improve patch coverage / FactoryとUserEntityのユニットテストを追加してパッチカバレッジを改善

[zigzagdev]

Motivation / 目的

After PR #497, Codecov still reported 69.89% patch coverage (target 85%). Investigation showed the following source files had no unit tests covering them.

PR #497 マージ後も Codecov のパッチカバレッジが 69.89%（目標 85%）のままでした。以下のソースファイルにユニットテストが不足していることが原因と判明しました。

What I have done / 実施内容

• SubscriptionFactoryTest: covers build() with null/string/DateTimeImmutable expires_at, string/DateTimeImmutable now (5 tests)
• UserEntityFactoryTest: covers build() creating correct UserEntity with Free and Premium subscriptions (2 tests)
• UserEntityTest: covers all getters including getFullName() (7 tests)
• UserViewModelFactoryTest: covers build() returning UserViewModel instance (1 test)

Test Results / テスト結果

All new tests pass. Previous 26 tests + 15 new = 41 unit tests total.

Closes part of #400

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.40%. Comparing base (ccc5e72) to head (a77ced6).
⚠️ Report is 1 commits behind head on feat/user_register.

Additional details and impacted files

@@                  Coverage Diff                  @@
##             feat/user_register     #498   +/-   ##
=====================================================
  Coverage                 55.40%   55.40%           
  Complexity                 1409     1409           
=====================================================
  Files                       102      102           
  Lines                      7039     7039           
=====================================================
  Hits                       3900     3900           
  Misses                     3139     3139           

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[zigzagdev]

coverage ok