[Bug]: Metadata is not applied when TaskArtifactUpdateEvent type is append

[flex-myeonghyeon]

What happened?

Description
When TaskArtifactUpdateEvent is handled by TaskManager with the type set to append,
the metadata included in the event is not applied to the existing artifact.

In the current behavior, even if an append event contains metadata,
the existing artifact’s metadata (existArtifact.metadata) remains unchanged,
and the metadata from TaskArtifactUpdateEvent is ignored.

Expected Behavior
When TaskArtifactUpdateEvent has the type append,
the metadata provided in the event should be merged into the existing artifact’s metadata.

Preserve existing existArtifact.metadata

Merge TaskArtifactUpdateEvent.metadata into it (adding or overriding keys as needed)

Suggested Fix
The append handling logic should merge
TaskArtifactUpdateEvent.metadata into existArtifact.metadata instead of ignoring it.

Relevant log output

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[kabir]

@flex-myeonghyeon I think this was fixed by #616 so I'll close it