feat: add support for roman numeral page numbers in TOC

[denis-samatov]

Summary

• Adds support for parsing Roman numeral page numbers (e.g. i, ii, iv, xii, IV) in Table of Contents structures.
• Resolves Add support for roman numeral page numbers (ex. ii, iv) #164: "Add support for roman numeral page numbers (ex. ii, iv)".
• Prevents documents whose introductory, preface, or TOC pages use Roman numerals from failing convert_page_to_int, being skipped by calculate_page_offset and add_page_offset_to_toc_json, and unnecessarily falling back to fallback search.

Changes

1. pageindex/utils.py:
   • Added roman_to_int(s: str) -> int | None with standard Roman numeral validation (case-insensitive, supporting i through m).
   • Updated convert_page_to_int(data) to attempt integer conversion first, and if ValueError, parse valid Roman numerals into integers.
2. tests/test_roman_numerals.py:
   • Added unit test suite covering valid Roman numerals (single digits, multi-digits, subtractive notation, mixed case, whitespace).
   • Added tests for invalid strings/types ensuring graceful handling without unexpected conversions.
   • Added integration test verifying convert_page_to_int on mixed Arabic and Roman numeral TOC structures.

Verification

• Ran pytest tests/test_roman_numerals.py: 41 passed.
• Ran full test suite: 471 passed, 54 skipped, 0 failures.

Closes #164

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