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
previousIndicesLookup = null;
            }
            maybeSetMappingPurgeFlag(previous, indexMetadata);
            return this;
        }
        private void maybeSetMappingPurgeFlag(@Nullable IndexMetadata previous, IndexMetadata updated) {
            if (checkForUnusedMappings) {
                return;
            }
            if (previous == null) {
                return;
            }
            final MappingMetadata mapping = previous.mapping();
            if (mapping == null) {
                return;
            }
            final MappingMetadata updatedMapping = updated.mapping();
            if (updatedMapping == null) {
                return;
            }
            if (mapping.getSha256().equals(updatedMapping.getSha256()) == false) {
                checkForUnusedMappings = true;
            }
        }
        private static boolean unsetPreviousIndicesLookup(IndexMetadata previous, IndexMetadata current) {
            if (previous == null) {
                return true;
            }
            if (previous.getAliases().equals(current.getAliases()) == false) {
                return true;