fix: don't allow BOM's item code at any level of child items (backport #27157)

[mergify]

Steps to reproduce:

1. Create BOM for Item A.
2. In child item, again add Item A.
3. Save.

Observed:
BOM gets saved.

Expected:
validation error.

Solution:
Never allow BOM's item at any level of exploded child items.

closes #26747

---

This is an automatic backport of pull request #27157 done by Mergify.

[mergify]

Cherry-pick of c07dce9 has failed:

On branch mergify/bp/version-13-hotfix/pr-27157
Your branch is up to date with 'origin/version-13-hotfix'.

You are currently cherry-picking commit c07dce940e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/manufacturing/doctype/bom/test_bom.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/manufacturing/doctype/bom/bom.py
	both modified:   erpnext/manufacturing/doctype/production_plan/test_production_plan.py
	both modified:   erpnext/manufacturing/doctype/work_order/test_work_order.py

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