Aggs: Add validation to Bucket script pipeline agg

[ivancea]

Closes #132272

Docs are explicit on what the bucket_script agg requires:

A parent pipeline aggregation which executes a script which can perform per bucket computations on specified metrics in the parent multi-bucket aggregation

But it's missing a validation.

I checked for the validate() method, but I couldn't find a sane way to check that the agg builder is a MultiBucket agg. There's an InternalMultiBucketAgg interface, but not one for the builders. I could add it, but I'm not sure it would be worth the time.

[elasticsearchmachine]

Hi @ivancea, I've created a changelog YAML for you.

[nik9000]

I'm ok with it. I'd ask @not-napoleon if he can think of a better way, but this is A way and that's better than a weird exception.

[elasticsearchmachine]

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

[not-napoleon]

If we don't want to support this (and I assume we don't, but that's more your call at this point), then I think this is a fine solution. As @nik9000 noted, it's better than the class cast exception.

I would like a yaml test with the reproducing query from the original issue. I see there's an Aggregator Test for it, which is good, but I would like both.

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	9.1	Commit could not be cherrypicked due to conflicts
❌	8.19	Commit could not be cherrypicked due to conflicts
❌	9.0	Commit could not be cherrypicked due to conflicts
❌	8.17	Commit could not be cherrypicked due to conflicts
❌	8.18	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 132320