Fix missing feature flag check in Vue vertical menu

What does this MR do and why?

This is a fix for the missing feature flag in the vertical menu. It's causing a double-scope menu when the FF is disabled.

6/7 add vertical navigation vue app

• Search Page vertical navigation search helper (merged)
• Search Page layout changes (merged)
• Move filters into their own component (merged)
• Fix search helper vertical navigation JSON method (merged)
• Search Page Vue vertical navigation (merged)
  • Fix missing feature flag check in Vue vertical menu (merged) 👈 this MR
  • Convert all Search Page RSpecs to support vertical search navigation

Screenshots or screen recordings

When Feature.disable(:search_page_vertical_nav)

before	after

How to set up and validate locally

1. checkout this branch
2. enable feature flag Feature.enable(:search_page_vertical_nav)
3. search any term on search page http://127.0.0.1:3000/search
4. disable feature flag Feature.disable(:search_page_vertical_nav)
5. search any term on search page http://127.0.0.1:3000/search
6. under both scenarios only one scope menu should be visible

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #293740 (closed)