test(sales): add coverage for zero-qty return rejection

Greptile flagged that the sales-side zero-qty-return fix had no dedicated
test proving the behavior - the existing suite happened to pass, but
nothing specifically asserted that an all-zero return is rejected while
a normal negative-qty return still succeeds.

Adds two tests covering the doctypes that rely entirely on this check
(no other guard covers them for a non-stock-effect return):
- Delivery Note return with qty 0 -> rejected
- Sales Invoice return with qty 0 (no update_stock) -> rejected

POS Invoice is not covered separately here since it always runs with
update_stock=1, which is already guarded by the pre-existing
validate_zero_qty_for_return_invoices_with_stock check regardless of
this fix.

(cherry picked from commit 732c884633)

# Conflicts:
#	erpnext/controllers/tests/test_sales_and_purchase_return.py

erpnext/controllers/tests/test_sales_and_purchase_return.py

@@ -0,0 +1,112 @@
+# Copyright (c) 2025, Frappe Technologies Pvt. Ltd. and Contributors
+# See license.txt
+
+import frappe
+
+from erpnext.tests.utils import ERPNextTestSuite
+
+
+class TestSalesAndPurchaseReturn(ERPNextTestSuite):
+	@staticmethod
+	def _cancel_and_delete(doctype, name):
+		if not frappe.db.exists(doctype, name):
+			return
+		doc = frappe.get_doc(doctype, name)
+		if doc.docstatus == 1:
+			doc.cancel()
+		frappe.delete_doc(doctype, name, force=1)
+
+	def test_sales_return_validates_against_original(self):
+		# Submitting a return Delivery Note runs validate_returned_items (Item / Packed Item lookups
+		# via frappe.get_all) and get_already_returned_items (qb GROUP BY of the returned qty) -- both
+		# converted from raw SQL here. Exercises them on both engines.
+		from erpnext.stock.doctype.delivery_note.mapper import make_sales_return
+		from erpnext.stock.doctype.delivery_note.test_delivery_note import create_delivery_note
+		from erpnext.stock.doctype.stock_entry.stock_entry_utils import make_stock_entry
+
+		se = make_stock_entry(item_code="_Test Item", target="_Test Warehouse - _TC", qty=20, basic_rate=100)
+		self.addCleanup(self._cancel_and_delete, "Stock Entry", se.name)
+
+		dn = create_delivery_note(qty=5)
+		self.addCleanup(self._cancel_and_delete, "Delivery Note", dn.name)
+
+		return_dn = make_sales_return(dn.name)
+		return_dn.insert()
+		return_dn.submit()
+		self.addCleanup(self._cancel_and_delete, "Delivery Note", return_dn.name)
+
+		self.assertEqual(return_dn.is_return, 1)
+		self.assertEqual(return_dn.items[0].qty, -5)
+
+	def test_purchase_invoice_zero_qty_return_is_rejected(self):
+		# A return with every item at qty 0 moves no stock and no value, so it must be
+		# rejected the same way a return with no items at all would be.
+		from erpnext.accounts.doctype.purchase_invoice.test_purchase_invoice import make_purchase_invoice
+
+		pi = make_purchase_invoice(qty=10)
+		self.addCleanup(self._cancel_and_delete, "Purchase Invoice", pi.name)
+
+		return_pi = make_purchase_invoice(
+			is_return=1,
+			return_against=pi.name,
+			qty=0,
+			do_not_save=True,
+		)
+
+		self.assertRaises(frappe.ValidationError, return_pi.save)
+
+	def test_purchase_invoice_item_name_only_zero_qty_return_is_rejected(self):
+		# Item Code is not mandatory on Purchase Invoice Item - a row can have only an
+		# item_name (e.g. a free-text/non-stock line). Such rows fall through to the
+		# item_name-only branch, which must also reject an all-zero-qty return instead
+		# of unconditionally treating the row as returned.
+		from erpnext.accounts.doctype.purchase_invoice.test_purchase_invoice import make_purchase_invoice
+
+		pi = make_purchase_invoice(item_name="_Test Item", qty=10, do_not_submit=True)
+		pi.items[0].item_code = ""
+		pi.save()
+		pi.submit()
+		self.addCleanup(self._cancel_and_delete, "Purchase Invoice", pi.name)
+
+		return_pi = make_purchase_invoice(
+			item_name="_Test Item",
+			is_return=1,
+			return_against=pi.name,
+			qty=0,
+			do_not_save=True,
+		)
+		return_pi.items[0].item_code = ""
+
+		self.assertRaises(frappe.ValidationError, return_pi.save)
+
+	def test_delivery_note_zero_qty_return_is_rejected(self):
+		# A return with every item at qty 0 moves no stock and no value, so it must be
+		# rejected the same way a return with no items at all would be.
+		from erpnext.stock.doctype.delivery_note.mapper import make_sales_return
+		from erpnext.stock.doctype.delivery_note.test_delivery_note import create_delivery_note
+		from erpnext.stock.doctype.stock_entry.stock_entry_utils import make_stock_entry
+
+		se = make_stock_entry(item_code="_Test Item", target="_Test Warehouse - _TC", qty=20, basic_rate=100)
+		self.addCleanup(self._cancel_and_delete, "Stock Entry", se.name)
+
+		dn = create_delivery_note(qty=5)
+		self.addCleanup(self._cancel_and_delete, "Delivery Note", dn.name)
+
+		return_dn = make_sales_return(dn.name)
+		return_dn.items[0].qty = 0
+
+		self.assertRaises(frappe.ValidationError, return_dn.insert)
+
+	def test_sales_invoice_zero_qty_return_is_rejected(self):
+		# Same rule for a standalone (non stock-affecting) Sales Invoice return: qty 0 on
+		# every row must be rejected, not silently accepted as a no-op credit note.
+		from erpnext.accounts.doctype.sales_invoice.test_sales_invoice import create_sales_invoice
+		from erpnext.controllers.sales_and_purchase_return import make_return_doc
+
+		si = create_sales_invoice(qty=10)
+		self.addCleanup(self._cancel_and_delete, "Sales Invoice", si.name)
+
+		return_si = make_return_doc(si.doctype, si.name)
+		return_si.items[0].qty = 0
+
+		self.assertRaises(frappe.ValidationError, return_si.save)