Fix flaky vector test (elastic#130732)

Closes elastic#130678

Author: jimczi

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml

@@ -621,9 +621,15 @@ setup:
       index:
         index: test_index
         id: "0"
+        refresh: true
         body:
           embedding: [ 0.5, 111.3, -13.0, 14.8, -156.0 ]
 
+  # wait and ensure that the mapping update is replicated
+  - do:
+      cluster.health:
+        wait_for_events: languid
+
   - do:
       indices.get_mapping:
         index: test_index