[pypdf] Add new project integration

[MR-SS]

Adds fuzzing integration for pypdf, a widely used Python PDF library with millions of downloads.

This integration implements a comprehensive Atheris fuzzer that exercises:

• PDF parsing via PdfReader
• Text extraction (in both layout and plain modes)
• Image extraction and stream decoding
• PDF metadata parsing
• PDF writing via PdfWriter

The build.sh script automatically fetches a diverse set of highly complex PDFs (from Mozilla's pdf.js test suite) to use as a seed corpus, ensuring high code coverage. Standard robustness exceptions (like ValueError, TypeError, AttributeError, OverflowError) are ignored to focus the fuzzer purely on Native crashes, OOM, and Timeouts.

Proof of Value:
During local testing with infra/helper.py, this fuzzer successfully discovered multiple unhandled edge cases and crashes in the upstream library within minutes, including:

• An unhandled AttributeError during dictionary casting
• An unhandled OverflowError during startxref parsing
• An Infinite Loop (Timeout) during text extraction of a malformed content stream

These findings have been reported to pypdf

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[github-actions]

MR-SS is integrating a new project:
- Main repo: https://github.com/py-pdf/pypdf
- Criticality score: 0.56828