Sub-issue: Implement PDF File Parser

[OmarMGaber]

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Description

Add a parser for .pdf files to extract text for indexing.

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What to Do

• Create a PDFParser struct that implements the Parser interface.
• Use a Go PDF parsing library like unidoc/unipdf or rsc.io/pdf to extract text.
• Ensure basic error handling for unsupported or encrypted PDFs.

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