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 routing;
        }
        IndexAbstraction result = getIndicesLookup().get(aliasOrIndex);
        if (result == null || result.getType() != IndexAbstraction.Type.ALIAS) {
            return routing;
        }
        Index writeIndexName = result.getWriteIndex();
        if (writeIndexName == null) {
            throw new IllegalArgumentException("alias [" + aliasOrIndex + "] does not have a write index");
        }
        AliasMetadata writeIndexAliasMetadata = index(writeIndexName).getAliases().get(result.getName());
        if (writeIndexAliasMetadata != null) {
            return resolveRouting(routing, aliasOrIndex, writeIndexAliasMetadata);
        } else {
            return routing;
        }
    }
    /**
     * Returns indexing routing for the given index.
     */
    
    
    public String resolveIndexRouting(@Nullable String routing, String aliasOrIndex) {
        if (aliasOrIndex == null) {
            return routing;
        }
        IndexAbstraction result = getIndicesLookup().get(aliasOrIndex);
        if (result == null || result.getType() != IndexAbstraction.Type.ALIAS) {
            return routing;
        }