Fix system data stream snapshot bug (elastic#124931)

This PR fixes a bug in the snapshot service that was allowing partial snapshots to restore features that use system data streams.

Author: JVerwolf

docs/changelog/124931.yaml

@@ -0,0 +1,5 @@
+pr: 124931
+summary: This PR fixes a bug whereby partial snapshots of system datastreams could be used to restore system features.
+area: "Snapshot/Restore"
+type: bug
+issues: []

server/src/internalClusterTest/java/org/elasticsearch/snapshots/SystemResourceSnapshotIT.java

@@ -893,7 +893,6 @@ public void testNoneFeatureStateOnCreation() {
         assertThat(snapshottedIndices, allOf(hasItem(regularIndex), not(hasItem(SystemIndexTestPlugin.SYSTEM_INDEX_NAME))));
     }
 
-    // TODO, Do we need to test this for Datastreams?
     /**
      * Ensures that if we can only capture a partial snapshot of a system index, then the feature state associated with that index is
      * not included in the snapshot, because it would not be safe to restore that feature state.

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -4124,7 +4124,15 @@ private SnapshotsInProgress createSnapshot(
 
                 if (featureSystemIndices.size() > 0 || featureAssociatedIndices.size() > 0 || featureDataStreamBackingIndices.size() > 0) {
 
-                    featureStates.add(new SnapshotFeatureInfo(featureName, List.copyOf(featureSystemIndices)));
+                    featureStates.add(
+                        new SnapshotFeatureInfo(
+                            featureName,
+                            List.copyOf(
+                                Stream.concat(featureSystemIndices.stream(), featureDataStreamBackingIndices.stream())
+                                    .collect(Collectors.toSet())
+                            )
+                        )
+                    );
                     indexNames.addAll(featureSystemIndices);
                     indexNames.addAll(featureAssociatedIndices);
                     indexNames.addAll(featureDataStreamBackingIndices);