Add assumption to ignore occasional test failures due to disconnected graphs (#14696)

Author: msokolov

lucene/core/src/test/org/apache/lucene/search/BaseVectorSimilarityQueryTestCase.java

@@ -491,6 +491,7 @@ public void testTimeout() throws IOException {
 
     try (Directory indexStore = getIndexStore(vectors);
         IndexReader reader = DirectoryReader.open(indexStore)) {
+      assumeTrue("graph is fully reachable", HnswUtil.graphIsRooted(reader, vectorField));
       IndexSearcher searcher = newSearcher(reader);
 
       // This query is cacheable, explicitly prevent it
@@ -504,7 +505,6 @@ public void testTimeout() throws IOException {
                   Float.NEGATIVE_INFINITY,
                   Float.NEGATIVE_INFINITY,
                   null));
-
       assertEquals(numDocs, searcher.count(query)); // Expect some results without timeout
 
       searcher.setTimeout(() -> true); // Immediately timeout