Malware scan retriggers on draft activation without content changes #778

When a draft with attachments is activated without modifying any attachment content, the malware scanner is retriggered unnecessarily.

Steps to reproduce:

1. Upload an attachment to a draft entity
2. Activate the draft
3. Wait for the scan status to become Clean
4. Enter draft mode again and activate without making any changes
5. The scan is retriggered (scannedAt timestamp changes, status resets to Scanning)

The root cause is in ModifyAttachmentEventFactory.handleExistingContentId() which treats a matching contentId with a non-null content stream as an update event. During draft activation the CAP runtime copies the stream reference even though the content is identical, so the factory incorrectly returns an update event instead of a doNothing event.