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
metadataWithIndices(oldVersionSystem, regularIndex);
    }
    public void testSystemIndexValidationAllRegular() {
        final IndexVersion random7xVersion = IndexVersionUtils.randomVersionBetween(
            random(),
            IndexVersion.V_7_0_0,
            IndexVersionUtils.getPreviousVersion(IndexVersion.V_8_0_0)
        );
        final IndexMetadata currentVersionSystem = buildIndexWithAlias(".system1", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);
        final IndexMetadata currentVersionSystem2 = buildIndexWithAlias(".system2", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);
        final IndexMetadata oldVersionSystem = buildIndexWithAlias(".oldVersionSystem", SYSTEM_ALIAS_NAME, null, random7xVersion, true);
        
        metadataWithIndices(currentVersionSystem, currentVersionSystem2, oldVersionSystem);
    }
    public void testSystemAliasValidationAllSystemSomeOld() {
        final IndexVersion random7xVersion = IndexVersionUtils.randomVersionBetween(
            random(),
            IndexVersion.V_7_0_0,
            IndexVersionUtils.getPreviousVersion(IndexVersion.V_8_0_0)
        );
        final IndexMetadata currentVersionSystem = buildIndexWithAlias(".system1", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);
        final IndexMetadata currentVersionSystem2 = buildIndexWithAlias(".system2", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);
        final IndexMetadata oldVersionSystem = buildIndexWithAlias(".oldVersionSystem", SYSTEM_ALIAS_NAME, null, random7xVersion, true);
        
        metadataWithIndices(currentVersionSystem, currentVersionSystem2, oldVersionSystem);
    }
    public void testSystemAliasValidationAll8x() {
        final IndexMetadata currentVersionSystem = buildIndexWithAlias(".system1", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);
        final IndexMetadata currentVersionSystem2 = buildIndexWithAlias(".system2", SYSTEM_ALIAS_NAME, null, IndexVersion.current(), true);