Backport: Fix bug in DocumentEvents2 sink implementation of the ContentControlBeforeContentUpdate event

[jozefizso]

Backports #454. Fixes #453.

Bug fix

The DocumentEvents2_SinkHelper.ContentControlBeforeContentUpdate method had a copy-paste bug
where it called Validate("ContentControlBeforeStoreUpdate") instead of
Validate("ContentControlBeforeContentUpdate").

Because the validation used the wrong event name, the event handler would never find
registered delegates for the ContentControlBeforeContentUpdate event. This caused
the COM event parameters (specifically the contentControl dispatch object) to be
released prematurely via ReleaseParamsArray on every call, instead of being properly
dispatched to user code. The result was a memory leak, as the underlying COM objects
were not properly managed by the event handler lifecycle.

The fix corrects the event name string passed to Validate() so that the
ContentControlBeforeContentUpdate event is properly matched and dispatched to subscribers.

Changes

• Fix the Validate() call in ContentControlBeforeContentUpdate to use the correct event name
• Add unit tests for DocumentEvents2_SinkHelper to verify the fix
• Update changelog for v1.9.8