…ed to time_series (elastic#137852)

This fixes an edge case introduced by elastic#137407. If a data stream is
created with the standard `index_mode`, and then the template changes
the mode to `time_series` without a `routing_path` and the data stream
is not rolled over, then the get data stream API and the get data stream
mappings API will fail with errors mentioning something like `failed to
apply settings java.lang.IllegalArgumentException:
[index.mode=time_series] requires a non-empty [index.routing_path]`.
Below is an example stack trace:

```
[2025-11-10T12:51:39,973][WARN ][o.e.c.s.IndexScopedSettings] [runTask-0] [.ds-quickstart-2-2025.11.10-000001] failed to apply settings java.lang.IllegalArgumentException: [index.mode=time_series] requires a non-empty [index.routing_path]
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.index.IndexMode$2.validateWithOtherSettings(IndexMode.java:156)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.index.IndexSettings$3.validate(IndexSettings.java:751)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.index.IndexSettings$3.validate(IndexSettings.java:745)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.Setting.get(Setting.java:589)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.Setting.get(Setting.java:561)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.index.mapper.MapperService.lambda$static$0(MapperService.java:134)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.Setting.innerGetRaw(Setting.java:657)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.Setting.getRaw(Setting.java:632)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.Setting$Updater.hasChanged(Setting.java:1317)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.AbstractScopedSettings$SettingUpdater.updater(AbstractScopedSettings.java:683)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.settings.AbstractScopedSettings.applySettings(AbstractScopedSettings.java:168)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.index.IndexSettings.updateIndexMetadata(IndexSettings.java:1495)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.cluster.metadata.DataStream.lambda$getEffectiveMappings$1(DataStream.java:520)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.indices.IndicesService.withTempIndexService(IndicesService.java:773)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.cluster.metadata.DataStream.getEffectiveMappings(DataStream.java:495)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.cluster.metadata.MetadataDataStreamsService.getEffectiveSettings(MetadataDataStreamsService.java:612)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.cluster.metadata.MetadataDataStreamsService.getEffectiveSettings(MetadataDataStreamsService.java:593)
        at org.elasticsearch.datastreams@9.3.0-SNAPSHOT/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.innerOperation(TransportGetDataStreamsAction.java:283)
        at org.elasticsearch.datastreams@9.3.0-SNAPSHOT/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.localClusterStateOperation(TransportGetDataStreamsAction.java:179)
        at org.elasticsearch.datastreams@9.3.0-SNAPSHOT/org.elasticsearch.datastreams.action.TransportGetDataStreamsAction.localClusterStateOperation(TransportGetDataStreamsAction.java:72)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.action.support.local.TransportLocalProjectMetadataAction.localClusterStateOperation(TransportLocalProjectMetadataAction.java:59)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.action.support.local.TransportLocalClusterStateAction.lambda$innerDoExecute$0(TransportLocalClusterStateAction.java:92)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.action.ActionRunnable$4.doRun(ActionRunnable.java:101)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:1076)
        at org.elasticsearch.server@9.3.0-SNAPSHOT/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
        at java.base/java.lang.Thread.run(Thread.java:1474)
```

(cherry picked from commit f2afb88)