feat: extract Markdown frontmatter metadata - #11615

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