Function parse_blockquote_atx_heading 

pub fn parse_blockquote_atx_heading(
    bq_content: &str,
) -> Option<(String, Option<String>)>

Parse an ATX heading written inside a blockquote’s inner text.

Blockquote headings (> ## Heading) are not detected by the main line-based heading parser, but they still produce valid fragment anchors. Strips the leading # marker, an optional CommonMark closing hash sequence, and any trailing {#custom-id}. Returns (clean_text, custom_id) or None when the blockquote content is not an ATX heading.