fix(pl): reject unknown plan product ids on upsert with 422.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/api/pl/plan.py

@@ -130,6 +130,7 @@ from app.services.pl_plan_service import (
     assert_contract_selectable_for_plan,
     apply_plan_product_fields,
     build_plan_product,
+    resolve_plan_product_id_for_upsert,
     calc_plan_total_amount,
     clear_draft_transport_for_plan,
     clear_hazard_cert_ready,
@@ -1645,7 +1646,10 @@ def update_plan(
     if payload.products is not None:
         existing_all = (
             db.query(PlPlanProduct)
-            .filter(PlPlanProduct.plan_id == id)
+            .filter(
+                PlPlanProduct.plan_id == id,
+                PlPlanProduct.tenant_no == tenant_no,
+            )
             .all()
         )
         existing_all_by_id = {row.id: row for row in existing_all}
@@ -1685,8 +1689,11 @@ def update_plan(
                     "legal_use_cert_url": legal_use_cert,
                 }
             )
-            row_id = (prod.id or "").strip() or None
-            existing = existing_all_by_id.get(row_id) if row_id else None
+            row_id, existing = resolve_plan_product_id_for_upsert(
+                plan_id=id,
+                payload_id=prod.id,
+                existing_by_id=existing_all_by_id,
+            )
             if existing is not None:
                 apply_plan_product_fields(
                     existing,
@@ -1706,6 +1713,7 @@ def update_plan(
                     order_num=idx,
                     user_id=user.id,
                     dept_id=user.dept_id,
+                    product_row_id=row_id,
                 )
                 db.add(pp)
                 kept_ids.add(pp.id)