feat: extract Markdown frontmatter metadata

[gyx09212214-prog]

Summary

Adds optional YAML frontmatter extraction to MarkdownToDocument.

When extract_frontmatter=True, Markdown files beginning with --- ... --- are parsed with PyYAML. Mapping values are added to Document.meta and the frontmatter block is removed before rendering document content. The default remains unchanged, so existing users keep frontmatter in the converted content unless they opt in.

Metadata precedence is ByteStream.meta < frontmatter < run(meta=...), matching the existing behavior where explicit runtime metadata can override source metadata. Date-like YAML scalars are kept as strings so common note fields like date: 2026-06-12 remain JSON-serializable metadata.

This is useful for Markdown/RAG ingestion pipelines where source notes carry fields like ticker, source, report date, author, or document id in frontmatter and downstream retrievers need them as metadata filters or citations.

Tests

• python -m pytest test/components/converters/test_markdown_to_document.py -q
• python -m py_compile haystack\components\converters\markdown.py test\components\converters\test_markdown_to_document.py
• python -m ruff check haystack\components\converters\markdown.py test\components\converters\test_markdown_to_document.py
• git diff --check

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[anakin87]

Thank you, it seems like a helpful addition.

I left some comments.
Please also fix the format error (running hatch run fmt should do trick) and add a release note.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/converters
markdown.py					117, 122-128, 146, 167-172
Project Total

This report was generated by python-coverage-comment-action

[anakin87]

Looks good!