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
"type": "long"
                                                },
                                                "rx": {
                                                    "type": "long"
                                                }
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            },
            "composed_of": ["custom_template"],
            "data_stream": {
            }
        }""";
    private static final String NON_TSDB_TEMPLATE = """
        {
            "index_patterns": ["k8s*"],
            "template": {
                "settings":{
                    "index": {
                        "number_of_replicas": 0,
                        "number_of_shards": 2
                    }
                },
                "mappings":{
                    "properties": {