Fix test to anticipate force merge failure (#126282)

This test had a copy paste mistake. When the cluster has only one data
node the replicas cannot be assigned so we end up with a force merge
error. In the case of the failure store this was not asserted correctly.

On the other hand, this test only checked for the existence of an error
and it was not ensuring that the current error is not the rollover error
that should have recovered. We make this test a bit more explicit.

Fixes: #126252

Author: gmarouli

modules/data-streams/src/internalClusterTest/java/org/elasticsearch/datastreams/lifecycle/ExplainDataStreamLifecycleIT.java

@@ -486,7 +486,9 @@ public void testExplainLifecycleForIndicesWithErrors() throws Exception {
                  * succeed, and there will always be an error in the error store. This behavior is subject to change in the future.
                  */
                 assertThat(response.getIndices().get(0).getError(), is(notNullValue()));
-                assertThat(response.getIndices().get(1).getError(), is(nullValue()));
+                assertThat(response.getIndices().get(0).getError().error(), containsString("Force merge request "));
+                assertThat(response.getIndices().get(1).getError(), is(notNullValue()));
+                assertThat(response.getIndices().get(1).getError().error(), containsString("Force merge request "));
             }
         });
     }

muted-tests.yml

@@ -387,9 +387,6 @@ tests:
 - class: org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshotsIntegTests
   method: testCreateAndRestoreSearchableSnapshot
   issue: https://github.com/elastic/elasticsearch/issues/119709
-- class: org.elasticsearch.datastreams.lifecycle.ExplainDataStreamLifecycleIT
-  method: testExplainLifecycleForIndicesWithErrors
-  issue: https://github.com/elastic/elasticsearch/issues/126252
 - class: org.elasticsearch.xpack.test.rest.XPackRestIT
   method: test {p0=transform/transforms_stats/Test get transform stats}
   issue: https://github.com/elastic/elasticsearch/issues/126270