Function wrap_with_content_source 

pub fn wrap_with_content_source(content: &str, source: ContentSource) -> String

Wrap content with its content source marker, sanitizing if necessary.

• ContentSource::ExternalContent: sanitizes via sanitize_external_content then wraps with [EXTERNAL CONTENT — …] / [END EXTERNAL CONTENT] delimiters.
• All other sources: content is returned unchanged.