fix timeline activity deleted field

[Weiko]

Fixes #6362

[greptile-apps]

PR Summary

The pull request introduces a filtering mechanism to remove invalid events from the timeline activities, ensuring only valid events are processed and displayed.

• EventList.tsx: Added filtering step using filterOutInvalidEvents to remove invalid events before grouping by month.
• filterOutInvalidEvents.ts: Introduced filterOutInvalidEvents utility function to filter out invalid events based on mainObjectMetadataItem.
• filterOutInvalidEvents.test.ts: Added unit tests for filterOutInvalidEvents covering various scenarios to ensure robustness.

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