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
assertDataStreams(metadata.getIndices(), (DataStreamMetadata) metadata.customs().get(DataStreamMetadata.TYPE));
    }
    public void testValidateDataStreamsForNullDataStreamMetadata() {
        Metadata metadata = Metadata.builder()
            .put(IndexMetadata.builder("foo-index").settings(settings(IndexVersion.current())).numberOfShards(1).numberOfReplicas(1))
            .build();
        try {
            assertDataStreams(metadata.getIndices(), DataStreamMetadata.EMPTY);
        } catch (Exception e) {
            fail("did not expect exception when validating a system without any data streams but got " + e.getMessage());
        }
    }
    public void testDataStreamAliases() {
        Metadata.Builder mdBuilder = Metadata.builder();
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-eu"));
        assertThat(mdBuilder.put("logs-postgres", "logs-postgres-eu", null, null), is(true));
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-us"));
        assertThat(mdBuilder.put("logs-postgres", "logs-postgres-us", null, null), is(true));
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-au"));
        assertThat(mdBuilder.put("logs-postgres", "logs-postgres-au", null, null), is(true));
        assertThat(mdBuilder.put("logs-postgres", "logs-postgres-au", null, null), is(false));
        Metadata metadata = mdBuilder.build();
        assertThat(metadata.dataStreamAliases().get("logs-postgres"), notNullValue());
        assertThat(
            metadata.dataStreamAliases().get("logs-postgres").getDataStreams(),
            containsInAnyOrder("logs-postgres-eu", "logs-postgres-us", "logs-postgres-au")
        );
    }
    public void testDataStreamReferToNonExistingDataStream() {
        Metadata.Builder mdBuilder = Metadata.builder();