fix: disallow mulitple SO with same Purchase Order No if not enabled in Settings (backport #36590)

[mergify]

This is an automatic backport of pull request #36590 done by Mergify.
Cherry-pick of 64614cd has failed:

On branch mergify/bp/version-13-hotfix/pr-36590
Your branch is ahead of 'origin/version-13-hotfix' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 64614cd915.
  (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/selling/doctype/sales_order/test_sales_order.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/stock/doctype/delivery_note/test_delivery_note.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[frappe-pr-bot]

🎉 This PR is included in version 13.52.12 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀