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
exception = expectThrows(IllegalArgumentException.class, () -> metadata.resolveWriteIndexRouting("1", "alias2"));
        assertThat(
            exception.getMessage(),
            is("index/alias [alias2] provided with routing value [1,2] that resolved to several routing values, rejecting operation")
        );
        exception = expectThrows(IllegalArgumentException.class, () -> metadata.resolveWriteIndexRouting(randomFrom("1", null), "alias4"));
        assertThat(
            exception.getMessage(),
            is("index/alias [alias4] provided with routing value [1,2] that resolved to several routing values, rejecting operation")
        );
        
        exception = expectThrows(IllegalArgumentException.class, () -> metadata.resolveWriteIndexRouting("1", "alias3"));
        assertThat(exception.getMessage(), is("alias [alias3] does not have a write index"));
        
        AliasMetadata.Builder aliasZeroBuilderTwo = AliasMetadata.builder("alias0");
        if (randomBoolean()) {
            aliasZeroBuilder.writeIndex(false);
        }
        IndexMetadata.Builder builder2 = IndexMetadata.builder("index2")
            .settings(Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, IndexVersion.current()))
            .numberOfShards(1)
            .numberOfReplicas(0)
            .putAlias(aliasZeroBuilderTwo.build())
            .putAlias(AliasMetadata.builder("alias1").routing("0").writeIndex(true).build())
            .putAlias(AliasMetadata.builder("alias2").writeIndex(true).build());
        Metadata metadataTwoIndices = Metadata.builder(metadata).put(builder2).build();
        
        assertThat("0", equalTo(metadataTwoIndices.resolveWriteIndexRouting("0", "alias1")));
    }
    public void testUnknownFieldClusterMetadata() throws IOException {