fix(stock): validate only the variant attributes that changed

Disabling an Item Attribute writes `disabled = 1` into every Item Variant
Attribute row, including the rows on the template. `validate_variant` runs
on every save and walks the whole attribute table, so any later save of an
existing variant re-checked its untouched rows against the now-disabled
template row and threw. `update_variants` hit the same wall, which made a
single template save fail once an attribute was disabled.

The flag exists to keep an attribute out of new variants, not to freeze the
variants that already use it. item.js only reads it to drop the attribute
from the variant creation dialog.

Skip rows that are unchanged since the last save. New and edited rows are
still checked, so a disabled attribute cannot be added to an existing
variant, and the same guard covers the sibling checks for attributes and
values that the template no longer offers.

(cherry picked from commit 25cd793617)

erpnext/stock/doctype/item/item.py

@@ -837,7 +837,17 @@ class Item(Document):
 				frappe.throw(_("Item {0} is not a template item.").format(frappe.bold(self.variant_of)))
 
 			if based_on == "Item Attribute":
+				previous_doc = self.get_doc_before_save()
+				saved_attributes = (
+					{(row.attribute, row.attribute_value) for row in previous_doc.attributes}
+					if previous_doc
+					else set()
+				)
+
 				for d in self.attributes:
+					if (d.attribute, d.attribute_value) in saved_attributes:
+						continue
+
 					if not frappe.db.exists(
 						"Item Variant Attribute", {"attribute": d.attribute, "parent": self.variant_of}
 					):