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
final Map<String, MappingMetadata> updatedMappingsByHash;
        if (mappingMetadata == null) {
            updatedMappingsByHash = mappingsByHash;
        } else {
            final MappingMetadata existingMapping = mappingsByHash.get(mappingMetadata.getSha256());
            if (existingMapping != null) {
                index = index.withMappingMetadata(existingMapping);
                updatedMappingsByHash = mappingsByHash;
            } else {
                updatedMappingsByHash = Maps.copyMapWithAddedEntry(mappingsByHash, mappingMetadata.getSha256(), mappingMetadata);
            }
        }
        final ImmutableOpenMap.Builder<String, IndexMetadata> builder = ImmutableOpenMap.builder(indices);
        builder.put(indexName, index);
        final ImmutableOpenMap<String, IndexMetadata> indicesMap = builder.build();
        for (var entry : updatedAliases.entrySet()) {
            List<IndexMetadata> aliasIndices = entry.getValue().stream().map(idx -> indicesMap.get(idx.getName())).toList();
            Builder.validateAlias(entry.getKey(), aliasIndices);
        }
        return new Metadata(
            clusterUUID,
            clusterUUIDCommitted,
            version,
            coordinationMetadata,
            transientSettings,
            persistentSettings,
            settings,
            hashesOfConsistentSettings,
            totalNumberOfShards + index.getTotalNumberOfShards(),
            totalOpenIndexShards + (index.getState() == IndexMetadata.State.OPEN ? index.getTotalNumberOfShards() : 0),