feat: extract and prepend PDF document metadata in PdfConverter

[SuperMarioYL]

Purpose

Fixes #1664. Extracts PDF document metadata (title, author, subject, keywords, creator, producer, creation/modification dates) and prepends it to the converted Markdown output.

Problem

The PdfConverter currently extracts only the text body of a PDF, discarding structured metadata from the document info dictionary. For research, legal, and LLM workflows, this metadata provides essential context for citation, provenance, and document management.

Solution

• Added _extract_pdf_metadata() that reads the PDF info dictionary via pdfminer.six (already a dependency) and decodes values from various encodings (UTF-16 BOM, UTF-8, latin-1)
• Added _format_pdf_date() to convert PDF date strings (D:YYYYMMDDHHmmSS) to human-readable YYYY-MM-DD HH:MM:SS format
• Metadata is formatted as a Markdown bullet list and prepended with a --- separator
• The title field on DocumentConverterResult is set when a PDF title is present
• Metadata is only prepended when the PDF has extractable text content, so scanned/image-only PDFs remain unaffected

Example output

- **Title:** Annual Report 2025
- **Author:** Jane Smith
- **Subject:** Financial Results
- **Keywords:** annual report, financials, 2025
- **Creator:** Test Suite
- **Producer:** MarkItDown Test
- **Created:** 2025-03-15 12:00:00

---

[body text follows]

Testing

• Added test_pdf_metadata.py with 7 test cases covering:
  • Full metadata extraction with all fields
  • Metadata ordering (appears before body)
  • Existing test PDFs still work correctly
  • Empty/scanned PDFs produce no metadata block
  • Title field set on result object
  • Date format conversion
• Updated expected output files for existing full-output comparison tests
• All 143 existing PDF-related tests continue to pass

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