A composable index template with no `template` defined in the body is mistakingly always assumed to not be a time series template. Even if it refers to a component template that has the `index.mode` setting set to `time_series` and the component template defines mappings with dimension fields or routing paths.

Incorrectly this causes to create a data stream with a tsdb index *without* time series start and end time settings set. The fact that this can happen is another bug and is being tracked #98833.

What should happen in this case is that a tsdb data stream is created with a valid tsdb backing index (with correctly generated time series start and end time settings).

This bug was found as part investigating #98792 and is the cause of that bug.
server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java
metConditions,
                silent,
                onlyValidate
            );
            case DATA_STREAM -> rolloverDataStream(
                currentState,
                (DataStream) indexAbstraction,
                rolloverTarget,
                createIndexRequest,
                metConditions,
                now,
                silent,
                onlyValidate,
                sourceIndexStats
            );
            default ->
                
                throw new IllegalStateException("unable to roll over type [" + indexAbstraction.getType().getDisplayName() + "]");
        };
    }
    public static void validateIndexName(ClusterState state, String index) {
        MetadataCreateIndexService.validateIndexName(index, state);
    }
    /**
     * Returns the names that rollover would use, but does not perform the actual rollover
     */
    public static NameResolution resolveRolloverNames(
        ClusterState currentState,
        String rolloverTarget,
        String newIndexName,