Add support for TaskArtifactUpdateEvents that use append = true

Currently the append = true property does not do anything. This event should not create a new artifact, but should update the last artifact and add a new part to the parts list.

Look for TaskArtifactUpdateEvents in the codebase, likely in an events or artifacts namespace. Identify where artifacts are created and updated. The change involves modifying the update logic to handle the 'append = true' property by adding a new part to an existing artifact's parts list instead of creating a new one. Check existing tests for artifact events to understand the expected behavior and add new tests for the append functionality.

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