Fix blob highlighting in search

What does this MR do?

Introduces a FoundBlob class and has parse_search_result return it, implementing the missing path message required by _file_highlight.html.haml

Are there points in the code the reviewer needs to double check?

Happy with FoundBlob instead of continuing to use OpenStruct?

This bug would have been a detectable NoMethodError if we weren't using OpenStruct, which is why I'm changing it.

EE needs fixing too, but we have https://gitlab.com/gitlab-org/gitlab-ee/issues/1567 to track that. They don't need to go in together.

Why was this MR needed?

Highlighting search results is currently broken

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30400 (closed)