[NL-to-ESQL] Add LLM-based query review

[pgayvallet]

To improve validation, we could try to run the query through the syntax checker and, in case of errors, ask the LLM to fix them.

If we "know" the query is targeting existing indices, we could even try to execute the query (e.g. with a limit 1) to see if it is valid.

The esql tooling exposes a validation function, which validate the syntax:

kibana/x-pack/plugins/inference/server/tasks/nl_to_esql/validate_esql_query.ts

Line 8 in 5c298a1

import { validateQuery } from '@kbn/esql-validation-autocomplete';

One issue might be extracting errors from those validators to a format the LLM could more easily understand.

If we have other way to check / validate the query, we should also experiment with those.