ESQL: data types transitioning from unsupported to supported fail queries

[alex-spies]

If a data type, like dense_vector, is currently unsupported, fields of that type can still be used in queries. The output will be populated with nulls, and you cannot use the field in expressions, but you can KEEP it.

Now, if we start supporting the new data type, new nodes will definitely start to actually extract the field from the index, and the output will have the appropriate type. But during rolling upgrades, and in CCS setups, remote nodes may still treat the data type as unsupported.

This means that FROM idx | KEEP unsupported_field will now start to fail even though it was just returning nulls before, because the FieldAttribute representing unsupported_field will now have a data type that may not even be present on older remote nodes - like 8.19 nodes, which are perfectly acceptable in CCS setups.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[alex-spies]

The long-term fix for this is #131108.

The scope of this issue are the dense_vector and aggregate_metric_double data types. As a short-term solution, we're going to consider them still unsupported in queries, unless the query uses either the knn function or the TS command, respectively.