Behavior: Removes the collection-to-content associations for each content item in the provided list. Note that this does NOT delete the collections themselves, only the associations between collections and content items.

Authorization: Requires an authenticated user via the class-level [Authorize] attribute.

Side Effects: Removes metadata associations between content items and collections. The collections and content items themselves remain intact.

Error Scenarios: Returns 400 if the input is null or the items list is empty. Returns 404 if the items list is null.