Fix group search for blobs bug

What does this MR do and why?

Bug: Group-level code searches were redirecting to Projects when global_search_code_enabled: false because the elasticsearch_code_scope setting was being applied to ALL search contexts instead of just global.

Fix: Use :group context instead of :global for scope calculation

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• set default_search_scope to 'system default'

  ApplicationSetting.last.update!(default_search_scope: 'system default')

• turn off global code search in Admin - Settings - Search - Visibililty and access controls
• advanced search or zoekt (or both) must be enabled for gdk
• set the following feature flags: search_scope_registry to true, work_item_scope_frontend to false
• Then perform a group search for blobs http://gdk.test:3000/search?group_id=22&scope=blobs&search=MymodifiedMessage

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