Migrate to data tiers API can configure data_content/data_hot for partially mounted indices

[andreidan]

Elasticsearch Version

7.17, 8.x

Installed Plugins

No response

Java Version

bundled

OS Version

Darwin

Problem Description

The migrate to data tiers API can sometimes configure an illegal value for the _tier_preference setting for partially mounted indices.
This can happen if two conditions are met:

1. if the partially mounted index doesn't have a _tier_preference configured
2. if the migrate to data tiers API doesn't perform any migration for this index (e.g. say this partially mounted index doesn't use the data custom attribute name for any allocation filtering, but either uses some other node attribute name or none at all)

In this case this piece of code kicks in to configure a default tier preference https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingService.java#L534

This is wrong, and we should configure the MountSearchableSnapshotRequest.Storage.defaultDataTiersPreference as the tier preference.

Relates to #84758

Steps to Reproduce

1. Mount an index - partial-test
2. Remove the _tier_preference using

PUT partial-test/_settings
{
  "index.routing.allocation.include._tier_preference": null
}

3. Call the migrate to data tiers routing API

POST /_ilm/migrate_to_data_tiers

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)