Fix missing types in markdown record output
Add extract_all_text helper that properly handles all inline renderer
elements (Text, Code_span, Inlines, Emphasis, etc.) instead of only
handling Text nodes. Fixes record fields losing their type annotations
in the unified code block output.
Cherry-picked from jonludlam/odoc@f303d8e69
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>