fix(stock): make get_incoming_value_for_serial_nos a staticmethod

It never references `self`. The deterministic-serial-value test added in #56249
called it as `get_incoming_value_for_serial_nos(None, sle, serial_nos)` — passing
None for self, which is fragile: a future `self.*` access would fail with an opaque
AttributeError. Declaring it @staticmethod makes the call honest
(`get_incoming_value_for_serial_nos(sle, serial_nos)`) and is backward compatible —
the method has no in-repo callers besides that test, and any `self.`-style call still
binds correctly to a staticmethod.

Addresses Greptile review feedback on #56249.

erpnext/stock/doctype/stock_ledger_entry/test_stock_ledger_entry.py

@@ -77,7 +77,7 @@ class TestStockLedgerEntry(ERPNextTestSuite, StockTestMixin):
 		mk_sle("MAT-SLE-TIE-B", 200)  # later/larger name -> deterministic winner
 
 		value = update_entries_after.get_incoming_value_for_serial_nos(
-			None, frappe._dict(company=company_a), [serial]
+			frappe._dict(company=company_a), [serial]
 		)
 		# the latest (creation/name desc) same-date SLE wins -> 200 on both engines
 		self.assertEqual(value, 200.0)

erpnext/stock/stock_ledger.py

@@ -1473,7 +1473,8 @@ class update_entries_after:
 			for item in sr.items:
 				item.db_update()
 
-	def get_incoming_value_for_serial_nos(self, sle, serial_nos):
+	@staticmethod
+	def get_incoming_value_for_serial_nos(sle, serial_nos):
 		# get rate from serial nos within same company
 		all_serial_nos = frappe.get_all(
 			"Serial No", fields=["purchase_rate", "name", "company"], filters={"name": ("in", serial_nos)}