fix(bom): allow zero qty for secondary items (Co-Product, By-Product, Scrap, Additional Finished Good)

Secondary output items in a BOM do not always guarantee output during
manufacture. The actual qty is only known when manufacturing completes,
so setting zero in the BOM is a valid way to express "output is
non-deterministic".

Changes:
- Remove `reqd: 1` from the qty field in BOM Secondary Item so that 0
  is accepted as an explicit value (non_negative constraint is kept, so
  negative values are still rejected).
- Relax validate_secondary_items() in bom.py to only reject qty that is
  None/missing, not qty that is explicitly 0.
- Add a qty event handler in bom.js that shows a blue informational
  alert when the user sets qty to 0, explaining that the actual output
  will be recorded at manufacture time.

Fixes https://github.com/frappe/erpnext/issues/55401

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

erpnext/manufacturing/doctype/bom/bom.js

@@ -1012,6 +1012,19 @@ frappe.ui.form.on("BOM Secondary Item", {
 	item_code(frm, cdt, cdn) {
 		const { item_code } = locals[cdt][cdn];
 	},
+
+	qty(frm, cdt, cdn) {
+		const row = locals[cdt][cdn];
+		if (flt(row.qty) === 0) {
+			frappe.show_alert({
+				message: __(
+					"Row #{0}: Qty is set to zero for {1}. The actual output will be entered at the time of manufacture.",
+					[row.idx, frappe.bold(row.item_code)]
+				),
+				indicator: "blue",
+			});
+		}
+	},
 });
 
 function trigger_process_loss_qty_prompt(frm, cdt, cdn, item_code) {

erpnext/manufacturing/doctype/bom/bom.py

@@ -373,9 +373,9 @@ class BOM(WebsiteGenerator):
 					).format(item.idx, get_link_to_form("Item", item.item_code))
 				)
 
-			if not item.qty:
+			if item.qty is None:
 				frappe.throw(
-					_("Row #{0}: Quantity should be greater than 0 for {1} Item {2}").format(
+					_("Row #{0}: Quantity is required for {1} Item {2}").format(
 						item.idx, item.secondary_item_type, get_link_to_form("Item", item.item_code)
 					)
 				)

erpnext/manufacturing/doctype/bom_secondary_item/bom_secondary_item.json

@@ -138,8 +138,7 @@
    "fieldtype": "Float",
    "in_list_view": 1,
    "label": "Qty",
-   "non_negative": 1,
-   "reqd": 1
+   "non_negative": 1
   },
   {
    "default": "0",