fix: parsing pdf error - new_cells as str has no "copy" (#3130)
Closes #3119.
### Testing
Parsing the provided PDF should be successful.
[testing_brochure_2.pdf](https://github.com/user-attachments/files/15518094/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]))
```