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/main/java/org/elasticsearch/cluster/metadata/Metadata.java
* @return {@code true} if an index exists with the same name and UUID as the given index object, {@code false} otherwise.
     */
    public boolean hasIndex(Index index) {
        IndexMetadata metadata = index(index.getName());
        return metadata != null && metadata.getIndexUUID().equals(index.getUUID());
    }
    /**
     * Checks whether an index abstraction (that is, index, alias, or data stream) exists (as of this {@link Metadata} with the given name.
     * @param index An index name that may or may not exist in the cluster.
     * @return {@code true} if an index abstraction with that name exists, {@code false} otherwise.
     */
    public boolean hasIndexAbstraction(String index) {
        return getIndicesLookup().containsKey(index);
    }
    public IndexMetadata index(String index) {
        return indices.get(index);
    }
    public IndexMetadata index(Index index) {
        IndexMetadata metadata = index(index.getName());
        if (metadata != null && metadata.getIndexUUID().equals(index.getUUID())) {
            return metadata;
        }
        return null;
    }
    
    public boolean hasIndexMetadata(final IndexMetadata indexMetadata) {
        return indices.get(indexMetadata.getIndex().getName()) == indexMetadata;
    }
    /**
     * Returns the {@link IndexMetadata} for this index.