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
ex = expectThrows(IllegalArgumentException.class, () -> metadata.resolveIndexRouting(null, "alias2"));
        assertThat(
            ex.getMessage(),
            is("index/alias [alias2] provided with routing value [1,2] that resolved to several routing values, rejecting operation")
        );
        ex = expectThrows(IllegalArgumentException.class, () -> metadata.resolveIndexRouting("1", "alias2"));
        assertThat(
            ex.getMessage(),
            is("index/alias [alias2] provided with routing value [1,2] that resolved to several routing values, rejecting operation")
        );
        IndexMetadata.Builder builder2 = IndexMetadata.builder("index2")
            .settings(Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current()))
            .numberOfShards(1)
            .numberOfReplicas(0)
            .putAlias(AliasMetadata.builder("alias0").build());
        Metadata metadataTwoIndices = Metadata.builder(metadata).put(builder2).build();
        
        IllegalArgumentException exception = expectThrows(
            IllegalArgumentException.class,
            () -> metadataTwoIndices.resolveIndexRouting("1", "alias0")
        );
        assertThat(exception.getMessage(), startsWith("Alias [alias0] has more than one index associated with it"));
    }
    public void testResolveWriteIndexRouting() {
        AliasMetadata.Builder aliasZeroBuilder = AliasMetadata.builder("alias0");
        if (randomBoolean()) {
            aliasZeroBuilder.writeIndex(true);
        }
        IndexMetadata.Builder builder = IndexMetadata.builder("index")
            .settings(Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current()))