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/test/java/org/elasticsearch/cluster/metadata/MetadataTests.java
Metadata.Builder builder = Metadata.builder();
        for (int i = 0; i < numAliases; i++) {
            String aliasName = "alias-" + i;
            for (int j = 0; j < numIndicesPerAlias; j++) {
                AliasMetadata.Builder alias = new AliasMetadata.Builder(aliasName);
                if (j == 0) {
                    alias.writeIndex(true);
                }
                String indexName = aliasName + "-" + j;
                builder.put(
                    IndexMetadata.builder(indexName)
                        .settings(settings(IndexVersion.current()))
                        .creationDate(randomNonNegativeLong())
                        .numberOfShards(1)
                        .numberOfReplicas(0)
                        .putAlias(alias)
                );
            }
        }
        Metadata metadata = builder.build();
        for (int i = 0; i < numAliases; i++) {
            String aliasName = "alias-" + i;
            Set<Index> result = metadata.aliasedIndices(aliasName);
            Index[] expected = IntStream.range(0, numIndicesPerAlias)
                .mapToObj(j -> aliasName + "-" + j)
                .map(name -> new Index(name, ClusterState.UNKNOWN_UUID))
                .toArray(Index[]::new);
            assertThat(result, containsInAnyOrder(expected));
        }