Function detect_code_blocks 

pub fn detect_code_blocks(content: &str) -> Vec<CodeBlockRegion>

Detects fenced code blocks in content.

Returns regions sorted by start position. Handles:

• Code blocks with language identifiers (rust, markdown, ~~~rust, ~~~markdown)
• Empty code blocks
• Multiple code blocks
• Both backtick (```) and tilde (~~~) syntax

Returns

A list of code block regions in order of appearance.