fix(pdf): remove NUL characters from parsed text

[Success6666]

Summary

• remove embedded NUL characters from parsed PDF text cells before they enter downstream document assembly
• sanitize character, word, and text-line cells for both standard and threaded Docling Parse backends
• preserve valid Unicode text, including superscript unit exponents

Issue resolved by this Pull Request:
Resolves #3858

Tests

• python -m pytest tests/test_backend_docling_parse.py -q (24 passed)
• changed-file prek hooks (Ruff, formatter, ty, Tach)
• converted the issue's 22-page source PDF with ConversionStatus.SUCCESS and verified zero NUL characters in both exported Markdown and the full serialized DoclingDocument

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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