fix: seed standard Item Groups under the existing tree root (backport #57616)

[mergify]

Closes #57581

install_fixtures always inserted "All Item Groups" as a parentless group. When the tree already had a root, ItemGroup.validate re-parented it, leaving a second group-root holding the standard groups while the real root held everything else.

Reproducible with the healthcare app on a non-English site: its after_install seeds the root as _("All Item Groups"), so a pt-BR site gets Todos os Grupos de Itens as the root before the setup wizard runs.

'Todos os Grupos de Itens' <- ''        (root)
  'Laboratory', 'Drug'
  'All Item Groups' (is_group=1)
    'Products', 'Raw Material', 'Services', 'Sub Assemblies', 'Consumable'

This predates #57390 — I reproduced it with the old translated-name lookup restored and got an identical tree, since _("All Item Groups") resolved to the same root. (The healthcare side is a separate bug: it should seed the untranslated name like install_fixtures does.)

Resolve the root once with get_root_of, falling back to the canonical English name on fresh installs, and use it for the root record's exists-guard and the standard groups' parent — same as Company.create_default_departments.

The patch merges an already-seeded "All Item Groups" into the root it sits under, lifting its children and repointing every link. It only fires when that node is a group whose parent is exactly the current root, and is a no-op on re-run.

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This is an automatic backport of pull request #57616 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   erpnext/patches.txt
	new file:   erpnext/patches/v16_0/merge_seeded_item_group_root.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/setup/doctype/item_group/test_item_group.py
	both modified:   erpnext/setup/setup_wizard/operations/install_fixtures.py

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