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/cluster/metadata/Metadata.java
private String clusterUUID;
        private boolean clusterUUIDCommitted;
        private long version;
        private CoordinationMetadata coordinationMetadata = CoordinationMetadata.EMPTY_METADATA;
        private Settings transientSettings = Settings.EMPTY;
        private Settings persistentSettings = Settings.EMPTY;
        private DiffableStringMap hashesOfConsistentSettings = DiffableStringMap.EMPTY;
        private final ImmutableOpenMap.Builder<String, IndexMetadata> indices;
        private final ImmutableOpenMap.Builder<String, Set<Index>> aliasedIndices;
        private final ImmutableOpenMap.Builder<String, IndexTemplateMetadata> templates;
        private final ImmutableOpenMap.Builder<String, Custom> customs;
        private SortedMap<String, IndexAbstraction> previousIndicesLookup;
        private final Map<String, ReservedStateMetadata> reservedStateMetadata;
        
        
        
        
        private boolean checkForUnusedMappings = true;
        private final Map<String, MappingMetadata> mappingsByHash;
        public Builder() {
            this(Map.of(), 0);
        }