This PR attempts to fix a memory issue, which resulted in errors like
this: https://github.com/Unstructured-IO/unstructured/issues/2931
The root cause seems to be in how ListItems are being combined, not in
how hashes or parent IDs are updated.
When `assign_and_map_hash_ids()` is called and elements (or elements'
metadata) do not have unique memory addresses, then updating the
parent_id of one element will also overwrite the parent_id of some other
element.
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Co-authored-by: cragwolfe <crag@unstructured.io>
