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/test/java/org/elasticsearch/datastreams/MetadataDataStreamRolloverServiceTests.java
} finally {
            testThreadPool.shutdown();
        }
    }
    public void testRolloverAndMigrateDataStream() throws Exception {
        Instant now = Instant.now().truncatedTo(ChronoUnit.SECONDS);
        String dataStreamName = "logs-my-app";
        IndexMode dsIndexMode = randomBoolean() ? null : IndexMode.STANDARD;
        final DataStream dataStream = new DataStream(
            dataStreamName,
            List.of(new Index(DataStream.getDefaultBackingIndexName(dataStreamName, 1, now.toEpochMilli()), "uuid")),
            1,
            null,
            false,
            false,
            false,
            false,
            dsIndexMode
        );
        ComposableIndexTemplate template = new ComposableIndexTemplate.Builder().indexPatterns(List.of(dataStream.getName() + "*"))
            .template(
                new Template(Settings.builder().put("index.mode", "time_series").put("index.routing_path", "uid").build(), null, null)
            )
            .dataStreamTemplate(new ComposableIndexTemplate.DataStreamTemplate(false, false))
            .build();
        Metadata.Builder builder = Metadata.builder();
        builder.put("template", template);
        Settings.Builder indexSettings = settings(IndexVersion.current()).put("index.hidden", true)
            .put(SETTING_INDEX_UUID, dataStream.getWriteIndex().getUUID());
        if (dsIndexMode != null) {