fix: parsing pdf error - new_cells as str has no "copy"

[christinestraub]

Closes #3119.

Testing

Parsing the provided PDF should be successful.

testing_brochure_2.pdf

filename = "testing_brochure_2.pdf"
with open(filename, "rb") as pdf_content:
    elements = partition_pdf(
        file=pdf_content,
        infer_table_structure=True,
        extract_image_block_types=["Image", "Table"],
        chunking_strategy="by_title",
        max_characters=1000,
        new_after_n_chars=3000,
        combine_text_under_n_chars=1000,
    )
print("\n\n".join([str(el) for el in elements]))

[cragwolfe]

Please add file to CI or in an ingest CI test (which has the added benefit of the outputs being browseable).

[cragwolfe]

Please add file to CI or in an ingest CI test (which has the added benefit of the outputs being browseable).

Per slack convo, this can be addressed in a separate PR.