Fix #107918: Weight Gradient tool paints over locked vertex groups

Exit the operator code if `DG_LOCK_WEIGHT` flag is set for active vertex
group to avoid painting over locked vertex group

@@ -745,0 +748,4 @@
+
+  int ret;
+
+  if (dg->flag & DG_LOCK_WEIGHT) {

@@ -146,0 +147,4 @@
+/**
+ * Return lock status of active vertex group.
+ */
+bool BKE_object_defgroup_active_is_locked(struct Object *ob);