Code search returns no results at intermediate group level

What does this MR do and why?

The main issue was that projects = Project.in_namespace(namespace_ids) does not consider the projects in the subgroups.

projects = Project.in_namespace(namespace_ids)
if projects.present? && !projects.id_not_in(authorized_projects).exists?
  projects

So it was just returning the projects from within the group, but not in the subgroups. So, for simplicity just removed this short-circuit

AI Summary

This code change improves how the search system handles user permissions when finding projects. The main improvements are:

Safety Enhancement: Added a check to prevent errors when no user is logged in - the system now returns an empty result instead of potentially crashing.

Code Simplification: Removed complex conditional logic that was checking project permissions in multiple ways and replaced it with a simpler, more direct approach. This makes the code easier to understand and maintain.

Better Project Filtering: When searching within specific projects, the system now properly handles cases where the requested projects don't exist, returning empty results instead of potentially showing incorrect data.

Improved Testing: Added comprehensive tests that cover different scenarios like searching globally, within groups, and for specific projects, including edge cases like non-existent projects and users without permissions.

The changes make the search functionality more reliable and predictable while reducing the complexity of the permission-checking logic. Users will experience more consistent search results, especially when they don't have access to certain projects or when searching for projects that don't exist.

References

https://gitlab.com/gitlab-com/request-for-help/-/work_items/4216

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Create this structure of projects.groups `GroupA -> SubGroupB(-> ProjectA) -> SubgroupC -> ProjectB
• Assign a direct membership of developer to a user to everything
• Keep all groups.projects private
• Perform a search in SubGroupB. You should see the search results from ProjectB

MR acceptance checklist

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