Fix efficient-filter test to satisfy LIMIT <= k contract

The efficient-filter test used LIMIT 5 with option='k=3', which the
branch rejects at push-down time via
VectorSearchQueryBuilder#validateLimitWithinK. Change LIMIT to 3 so the
query passes validation and actually exercises the k-NN execution path.

The discriminator is preserved: query near TX with WHERE state='CA' and
k=3 would return zero rows under POST filtering (the three nearest
candidates are all TX and get filtered out) and returns exactly {4,5,6}
under efficient filtering, which navigates HNSW toward the CA cluster.

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

integ-test/src/test/java/org/opensearch/sql/sql/VectorSearchExecutionIT.java

@@ -166,7 +166,7 @@ public void testEfficientFilterReturnsOnlyMatchingDocs() throws IOException {
                 + "', field='embedding', "
                 + "vector='[1.0, 1.0]', option='k=3,filter_type=efficient') AS v "
                 + "WHERE v.state = 'CA' "
-                + "LIMIT 5");
+                + "LIMIT 3");
 
     assertRowIdsEqual(result, "4", "5", "6");
   }