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.
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
assertOK(response);
        assertThat(entityAsMap(response).get("_index"), equalTo(secondBackingIndex));
        var searchRequest = new Request("GET", "k8s/_search");
        searchRequest.setJsonEntity("""
            {
                "query": {
                    "range":{
                        "@timestamp":{
                            "gte": "now-7d",
                            "lte": "now+7d"
                        }
                    }
                },
                "sort": [
                    {
                        "@timestamp": {
                            "order": "desc"
                        }
                    }
                ]
            }
            """);
        response = client().performRequest(searchRequest);
        assertOK(response);
        responseBody = entityAsMap(response);
        try {
            assertThat(ObjectPath.evaluate(responseBody, "hits.total.value"), equalTo(10));
            assertThat(ObjectPath.evaluate(responseBody, "hits.total.relation"), equalTo("eq"));
            assertThat(ObjectPath.evaluate(responseBody, "hits.hits.0._index"), equalTo(secondBackingIndex));
            assertThat(ObjectPath.evaluate(responseBody, "hits.hits.1._index"), equalTo(firstBackingIndex));