fix(document-extractor): add PDF text fallback

[luochen211]

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Related Issue

Closes #188
Refs langgenius/dify#37488

Summary

• Add a pypdf fallback when PDFium extracts only empty or whitespace text from a PDF.
• Keep PDFium as the primary parser and preserve the original empty result if the fallback also has no text or cannot parse the file.
• Add regression coverage for the empty-PDFium-output path and declare pypdf as a direct dependency.

Validation

• uv lock --check
• uv run ruff format --check src/graphon/nodes/document_extractor/node.py tests/nodes/document_extractor/test_dispatch.py
• uv run ruff check src/graphon/nodes/document_extractor/node.py tests/nodes/document_extractor/test_dispatch.py
• uv run ty check
• uv run pytest tests/nodes/document_extractor
• uv run pytest

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation

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[luochen211]

I have read the CLA Document and I hereby sign the CLA

[luochen211]

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