Re-enable vector.rescoring.directio (elastic#128898)

This commit re-enables vector.rescoring.directio, which was previously disabled because of issues with serverless.

resolves elastic#128829

Author: ChrisHegarty

muted-tests.yml

@@ -478,9 +478,6 @@ tests:
 - class: org.elasticsearch.index.codec.vectors.es818.ES818BinaryQuantizedVectorsFormatTests
   method: testSimpleOffHeapSizeFSDir
   issue: https://github.com/elastic/elasticsearch/issues/128800
-- class: org.elasticsearch.index.store.DirectIOIT
-  method: testDirectIOUsed
-  issue: https://github.com/elastic/elasticsearch/issues/128829
 - class: org.elasticsearch.packaging.test.DockerTests
   method: test150MachineDependentHeap
   issue: https://github.com/elastic/elasticsearch/issues/128120

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOLucene99FlatVectorsReader.java

@@ -57,7 +57,7 @@
 @SuppressForbidden(reason = "Copied from lucene")
 public class DirectIOLucene99FlatVectorsReader extends FlatVectorsReader implements OffHeapStats {
 
-    private static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "false"));
+    private static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "true"));
 
     private static final long SHALLOW_SIZE = RamUsageEstimator.shallowSizeOfInstance(DirectIOLucene99FlatVectorsReader.class);