Fix(Demerge_pdfs): Fix crash on empty hardcoded PDF, PyPDF2 deprecations, add multi-mode splitting & CLI (#527)

[Prayas340]

Description

This Pull Request refactors and modernizes the Demerge_pdfs project:

• Fixes runtime crash caused by hardcoding the empty 0-byte Python.pdf file, adding interactive path prompts with validation as well as full command-line arguments.
• Modernizes PDF handling with a unified PDFAdapter compatible with modern pypdf and PyPDF2 (v3+ and legacy versions).
• Adds 4 splitting modes:
  1. Custom Interactive Wizard: Step-by-step assistant with remaining page counters and bounds validation.
  2. Equal Chunk Splitting: Splits PDF into parts of N pages each.
  3. Page Ranges Extraction: Extracts custom ranges (e.g. 1-5, 8, 12-20) or individual pages.
  4. Burst Mode: Splits every page into its own individual PDF document.
• Adds CLI automation support with argparse flags (-i, -m, -p, -r, -o, --prefix).
• Implements filename sanitization and safe context managers (with open(...)).
• Adds automated test suite in test_demerging_pdfs.py (5/5 tests passing).
• Adds requirements.txt and updates README.md with usage instructions and examples.

Fixes #527

• Issue_no: Bug & Feature(Demerge_pdfs): Fix crash on empty hardcoded PDF, PyPDF2 deprecations, and add multi-mode splitting & CLI #527
• Not applicable

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation Update

Project

• Your Project Name: Demerge_pdfs

• Short Description: A robust, multi-mode Python utility to split, extract page ranges, or burst PDF documents using an interactive wizard or CLI automation.

Checklist:

• I have named my files and folder, according to this project's guidelines.
• My code follows the style guidelines of this project.
• I have commented on my code, particularly in hard-to-understand areas.
• I have created a helpful and easy to understand README.md, according to the given README_TEMPLATE.
• My changes do not produce any warnings.
• Added automated test suite test_demerging_pdfs.py.
• Added requirements.txt.