API - fix searching in group/project specified by path

What does this MR do?

It fixes searching using a new API endpoint when a project/group is specified by its full_path.

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

Why was this MR needed?

When a project/group is specified by its full_path the path is sent as a param into SearchService although it expects id.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42937 (closed)