Fix bug rendering BlobType markdown data

What does this MR do?

This MR fixes a bug introduced in !24856 (merged) when rendering markdown data through the snippet blob GraphQL endpoint.

It also renames the attributes highlightedData and plainHighlightedData attributes to make them more readable

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content