refactor(postgres): port Cost Center doctype queries to the query builder

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

erpnext/accounts/doctype/cost_center/cost_center.py

@@ -84,10 +84,10 @@ class CostCenter(NestedSet):
 		return frappe.db.get_value("GL Entry", {"cost_center": self.name})
 
 	def check_if_child_exists(self):
-		return frappe.db.sql(
-			"select name from `tabCost Center` where \
-			parent_cost_center = %s and docstatus != 2",
-			self.name,
+		return frappe.get_all(
+			"Cost Center",
+			filters={"parent_cost_center": self.name, "docstatus": ["!=", 2]},
+			pluck="name",
 		)
 
 	def if_allocation_exists_against_cost_center(self):