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
(ComposableIndexTemplateMetadata) this.customs.get(ComposableIndexTemplateMetadata.TYPE)
            ).map(itmd -> new HashMap<>(itmd.indexTemplates())).orElse(new HashMap<>());
            existingTemplates.put(name, indexTemplate);
            this.customs.put(ComposableIndexTemplateMetadata.TYPE, new ComposableIndexTemplateMetadata(existingTemplates));
            return this;
        }
        public Builder removeIndexTemplate(String name) {
            
            Map<String, ComposableIndexTemplate> existingTemplates = Optional.ofNullable(
                (ComposableIndexTemplateMetadata) this.customs.get(ComposableIndexTemplateMetadata.TYPE)
            ).map(itmd -> new HashMap<>(itmd.indexTemplates())).orElse(new HashMap<>());
            existingTemplates.remove(name);
            this.customs.put(ComposableIndexTemplateMetadata.TYPE, new ComposableIndexTemplateMetadata(existingTemplates));
            return this;
        }
        public DataStream dataStream(String dataStreamName) {
            return dataStreamMetadata().dataStreams().get(dataStreamName);
        }
        public Builder dataStreams(Map<String, DataStream> dataStreams, Map<String, DataStreamAlias> dataStreamAliases) {
            previousIndicesLookup = null;
            // Only perform data stream validation only when data streams are modified in Metadata:
            for (DataStream dataStream : dataStreams.values()) {
                dataStream.validate(indices::get);
            }
            this.customs.put(
                DataStreamMetadata.TYPE,
                new DataStreamMetadata(
                    ImmutableOpenMap.<String, DataStream>builder().putAllFromMap(dataStreams).build(),
                    ImmutableOpenMap.<String, DataStreamAlias>builder().putAllFromMap(dataStreamAliases).build()
                )