Load VFS storages, idle checkers and audit logs through bulk get

vfs_storage_loader (global_search_vfs_storages), idle_checker_loader (idle_checker.admin_search) and audit_log_loader (audit_log.global_search) read through SUPERADMIN-gated global searches. These entities are close to admin-only, but a loader is reached from the nodes that reference them, so it answers per id. Switch each to an entity partial bulk get.

## Success Criteria
- [ ] all three loaders read through entity partial bulk gets
- [ ] an unreadable id errors only its own field; a missing id resolves to None
- [ ] pants test passes for affected packages

JIRA Issue: BA-7893

Start with vfs_storage_loader, idle_checker_loader, and audit_log_loader, tracing their current global_search_vfs_storages, idle_checker.admin_search, and audit_log.global_search calls. Compare nearby entity partial bulk-get implementations, then run the affected packages' Pants tests; done means all three use partial bulk gets, unreadable IDs fail only their own fields, and missing IDs resolve to None.

Written by the indexing model from the issue text.