Move the merge as version heirarchy checks to the bulk edit method

src/documents/bulk_edit.py

@@ -702,7 +702,15 @@ def merge_as_versions(
         )
         documents_by_id = {document.id: document for document in documents}
 
+        if any(document.root_document_id is not None for document in documents):
+            raise ValueError("Only top-level documents can be merged as versions.")
+
         source_ids = [doc_id for doc_id in doc_ids if doc_id != root_document_id]
+        if Document.global_objects.filter(root_document_id__in=source_ids).exists():
+            raise ValueError(
+                "Documents with existing versions cannot be merged into another document.",
+            )
+
         root_document = documents_by_id[root_document_id]
         next_version_index = (
             Document.global_objects.filter(