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
"index, alias, and data stream names need to be unique, but the following duplicates were found [data "
                    + "stream ["
                    + dataStreamName
                    + "] conflicts with index]"
            )
        );
    }
    public void testBuilderRejectsDataStreamThatConflictsWithAlias() {
        final String dataStreamName = "my-data-stream";
        IndexMetadata idx = createFirstBackingIndex(dataStreamName).putAlias(AliasMetadata.builder(dataStreamName).build()).build();
        Metadata.Builder b = Metadata.builder().put(idx, false).put(newInstance(dataStreamName, List.of(idx.getIndex())));
        IllegalStateException e = expectThrows(IllegalStateException.class, b::build);
        assertThat(
            e.getMessage(),
            containsString(
                "index, alias, and data stream names need to be unique, but the following duplicates were found ["
                    + dataStreamName
                    + " (alias of ["
                    + DataStream.getDefaultBackingIndexName(dataStreamName, 1)
                    + "]) conflicts with data stream]"
            )
        );
    }
    public void testBuilderRejectsAliasThatRefersToDataStreamBackingIndex() {
        final String dataStreamName = "my-data-stream";
        final String conflictingName = DataStream.getDefaultBackingIndexName(dataStreamName, 2);
        IndexMetadata idx = createFirstBackingIndex(dataStreamName).putAlias(new AliasMetadata.Builder(conflictingName)).build();
        Metadata.Builder b = Metadata.builder().put(idx, false).put(newInstance(dataStreamName, List.of(idx.getIndex())));
        AssertionError e = expectThrows(AssertionError.class, b::build);
        assertThat(e.getMessage(), containsString("aliases [" + conflictingName + "] cannot refer to backing indices of data streams"));