feat: Add unit tests for events-players spec

[rubenvdlinde]

Summary

• Add 21 unit tests verifying the events-players spec requirements
• CharacterServiceTest: 8 tests for event effect application (EVT-008, EVT-020-023)
• EventTest: 7 tests for Event entity serialization (EVP-010-012)
• PlayerTest: 6 tests for Player entity serialization (EVP-015, PLR-001-008)
• OpenSpec change artifacts documenting the implementation

Test plan

• All 21 unit tests pass locally via php vendor/bin/phpunit
• Tests cover event effect stacking, missing event graceful handling, effect application order
• Tests verify Event uses title not name (EVP-010)
• Tests verify Player skeletal model with 3 fields (EVP-015)