[ML] Fix Non-Deterministic Training Set Selection in RegressionIT testTwoJobsWithSameRandomizeSeedUseSameTrainingSet (#138063)

The test testTwoJobsWithSameRandomizeSeedUseSameTrainingSet fails intermittently because documents may be processed in different orders during reindexing. Since we use an online reservoir sampling algorithm, this order actually matters. To ensure deterministic reindexing of the document sequence, both the number of shards and the number of segments must be 1.

This PR fixes the test by creating the source index with only 1 segment. This ensures deterministic document order during reindexing, resulting in consistent ID assignments and training set selection when using the same seed.

Fixes #117805

Author: valeriy42

muted-tests.yml

@@ -58,9 +58,6 @@ tests:
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=transform/transforms_reset/Test reset running transform}
   issue: https://github.com/elastic/elasticsearch/issues/117473
-- class: org.elasticsearch.xpack.ml.integration.RegressionIT
-  method: testTwoJobsWithSameRandomizeSeedUseSameTrainingSet
-  issue: https://github.com/elastic/elasticsearch/issues/117805
 - class: org.elasticsearch.packaging.test.ArchiveTests
   method: test44AutoConfigurationNotTriggeredOnNotWriteableConfDir
   issue: https://github.com/elastic/elasticsearch/issues/118208

x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/RegressionIT.java

@@ -361,6 +361,9 @@ public void testStopAndRestart() throws Exception {
     public void testTwoJobsWithSameRandomizeSeedUseSameTrainingSet() throws Exception {
         String sourceIndex = "regression_two_jobs_with_same_randomize_seed_source";
         indexData(sourceIndex, 100, 0);
+        // Force merge to single segment to ensure deterministic _doc sort order during reindexing
+        // Without this, multiple segments or segment merges can cause non-deterministic document processing order
+        client().admin().indices().prepareForceMerge(sourceIndex).setMaxNumSegments(1).setFlush(true).get();
 
         String firstJobId = "regression_two_jobs_with_same_randomize_seed_1";
         String firstJobDestIndex = firstJobId + "_dest";