Prevent unnecessary work item refetches by updating cache correctly

Hichem dahirequested to merge
gitlab-community/gitlab-org/gitlab:577802-prevent-refetching-through-safe-cache-modification into master

What does this MR do and why?

Add proper cache modification to prevent parent work item refetches

  • Add merge policies for userPermissions and namespace.userPermissions
  • Keep cache.modify to update specific fields instead of the whole query
  • Remove refs and toReference usage when updating the cache

References

#577802

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Screencast_from_2025-10-20_11-32-53 Screencast_from_2025-10-20_15-01-59

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