🧪 [Testing Improvement] Add EventService Tests

[longsizhuo]

🎯 What: The EventService class was missing test coverage. As it acts as a thin wrapper around EventRepository and EventInterestRepository, the missing tests left a gap in verifying the delegation logic.
📊 Coverage: Added 12 unit tests using Mockito to verify all public methods in EventService. This covers CRUD operations delegated to EventRepository and interest-related operations delegated to EventInterestRepository, including handling of empty collections for batch operations.
✨ Result: 100% method coverage achieved for EventService, ensuring that any future refactoring won't break the delegation logic and dependencies.

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PR created automatically by Jules for task 11754486694024451059 started by @longsizhuo

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Pull request overview

This PR adds missing unit test coverage for EventService, validating that each public method correctly delegates to EventRepository and EventInterestRepository without requiring a database.

Changes:

• Added a new Mockito-based test suite covering all public EventService methods.
• Added delegation assertions for CRUD operations and “interest” operations, including an empty-collection batch case.

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