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
+ "]; alias must have the same is_hidden setting "
                        + "on all indices"
                );
            }
            // Validate system status
            final Map<Boolean, List<IndexMetadata>> groupedBySystemStatus = indexMetadatas.stream()
                .collect(Collectors.groupingBy(IndexMetadata::isSystem));
            // If the alias has either all system or all non-system, then no more validation is required
            if (isNonEmpty(groupedBySystemStatus.get(false)) && isNonEmpty(groupedBySystemStatus.get(true))) {
                final List<String> newVersionSystemIndices = groupedBySystemStatus.get(true)
                    .stream()
                    .filter(i -> i.getCreationVersion().onOrAfter(IndexNameExpressionResolver.SYSTEM_INDEX_ENFORCEMENT_INDEX_VERSION))
                    .map(i -> i.getIndex().getName())
                    .sorted() // reliable erro
                    .toList();
                if (newVersionSystemIndices.isEmpty() == false) {
                    final List<String> nonSystemIndices = groupedBySystemStatus.get(false)
                        .stream()
                        .map(i -> i.getIndex().getName())
                        .sorted() // reliable erro
                        .toList();
                    throw new IllegalStateException(
                        "alias ["
                            + aliasName
                            + "] refers to both system indices "
                            + newVersionSystemIndices
                            + " and non-system indices: "
                            + nonSystemIndices
                            + ", but aliases must refer to either system or"
                            + " non-system indices, not both"