Add PDFKit-backed PDF converter, update docs, tests, and CI matrix

[etherlink-intern]

Motivation

• Implement embedded-text PDF extraction on Apple platforms so PDFs return Markdown when PDFKit is available.
• Keep the documentation and README in sync with the new PDF capability and document remaining work (OCR fallback and cross-platform support).
• Run tests and smoke tests across macOS and Linux in CI while making PDF tests conditional on PDFKit availability.

Description

• Add PDFConverter (compiled behind #if canImport(PDFKit)) that extracts embedded page text, returns Markdown, and emits pageCount and extractedTextPageCount metadata.
• Register PDFConverter in MarkItDown.defaultConverters so PDFs are handled by the default pipeline.
• Add PDF fixtures (Tests/Fixtures/sample.pdf, Tests/Fixtures/empty.pdf) and an expected output (Tests/Expected/sample.md) and extend tests to conditionally exercise PDF conversion when PDFKit is available.
• Update Scripts/smoke-test.sh to run or expect PDF behavior based on PDFKit availability, update documentation in README.md and Docs/NativeConverterBackends.md, and expand the GitHub Actions workflow to a matrix with macos-15 and ubuntu-latest.

Testing

• Ran the unit test suite with swift test on macOS where PDFKit is available and the PDF tests were exercised and passed.
• Ran the CLI fixture smoke tests with Scripts/smoke-test.sh on macOS (PDF cases exercised) and on Ubuntu (PDF cases asserted as unsupported), and the smoke tests passed.
• CI is configured to run the same swift test and Scripts/smoke-test.sh checks across the new OS matrix via the updated workflow.

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Codex Task