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
if (part.indices == updatedIndices
                && builder.dataStreamMetadata() == part.custom(DataStreamMetadata.TYPE, DataStreamMetadata.EMPTY)) {
                builder.previousIndicesLookup = part.indicesLookup;
            }
            return builder.build(true);
        }
    }
    public static final TransportVersion MAPPINGS_AS_HASH_VERSION = TransportVersion.V_8_1_0;
    public static Metadata readFrom(StreamInput in) throws IOException {
        Builder builder = new Builder();
        builder.version = in.readLong();
        builder.clusterUUID = in.readString();
        builder.clusterUUIDCommitted = in.readBoolean();
        builder.coordinationMetadata(new CoordinationMetadata(in));
        builder.transientSettings(readSettingsFromStream(in));
        builder.persistentSettings(readSettingsFromStream(in));
        if (in.getTransportVersion().onOrAfter(TransportVersion.V_7_3_0)) {
            builder.hashesOfConsistentSettings(DiffableStringMap.readFrom(in));
        }
        final Function<String, MappingMetadata> mappingLookup;
        if (in.getTransportVersion().onOrAfter(MAPPINGS_AS_HASH_VERSION)) {
            final Map<String, MappingMetadata> mappingMetadataMap = in.readMapValues(MappingMetadata::new, MappingMetadata::getSha256);
            if (mappingMetadataMap.size() > 0) {
                mappingLookup = mappingMetadataMap::get;
            } else {
                mappingLookup = null;
            }
        } else {
            mappingLookup = null;
        }