Returning correct index mode from get data streams api (elastic#137646) (elastic#137650)

(cherry picked from commit 5230e10)

Author: masseyke

modules/data-streams/src/main/java/org/elasticsearch/datastreams/action/TransportGetDataStreamsAction.java

@@ -224,6 +224,12 @@ static IndexMode resolveMode(
                 indexMode = Enum.valueOf(IndexMode.class, rawMode.toUpperCase(Locale.ROOT));
             }
         }
+        if (indexMode == null) {
+            String rawMode = settings.get(IndexSettings.MODE.getKey());
+            if (rawMode != null) {
+                indexMode = Enum.valueOf(IndexMode.class, rawMode.toUpperCase(Locale.ROOT));
+            }
+        }
         return indexMode;
     }
 

x-pack/plugin/logsdb/src/yamlRestTest/resources/rest-api-spec/test/15_default.yml

@@ -0,0 +1,19 @@
+'default index mode':
+  - requires:
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: PUT
+          path: /{index}
+          capabilities: [ logsdb_index_mode ]
+      reason: "Support for 'logsdb' index mode capability required"
+
+  - do:
+      indices.create_data_stream:
+        name: logs-test-1
+
+  - do:
+      indices.get_data_stream:
+        name: logs-test-1
+
+  - match: { data_streams.0.indices.0.index_mode: logsdb}
+  - match: { data_streams.0.index_mode: logsdb}