[ES|QL] Suggest only aggregation functions for SPARKLINE first argument - approach 2

[momovdg]

part of: #265936

Summary

This PR addresses only the autocomplete functionality for the SPARKLINE function.

Problem

ES|QL's SPARKLINE function has a special first parameter that must always be an aggregation expression (e.g. AVG(bytes)). Elasticsearch signals this via hint: { kind: 'aggregation' } on the param definition, but Kibana's autocomplete ignored that hint entirely:

Inside STATS SPARKLINE(|, ...), all scalar functions (SUBSTRING, ABS, etc.) were suggested alongside aggregations, plus every field in the index — none of which are valid input.
The existing hint handler (buildSuggestionsFromHints) only dispatched on hint.entityType (used by inference_endpoint), so a kind-only hint was a no-op.

Fix

Extends the existing hint dispatcher to also key off hint.kind. The hint produces an exclusive suggestion set, so fields and other functions are skipped automatically — no flag plumbing needed (compared to this approach #268454):

• ParameterHint.entityType → optional, with the three caller-side guards.
• New kindBasedResolvers map in parameters_from_hints.ts, keyed by hint.kind. Adding a kind = adding one resolver entry.
• aggregationKindResolver uses getAllFunctions({ type: [AGG, TIME_SERIES_AGG] }) then applies the same predicates the standard path uses (location, license, ignored, paramDefinitions return types) and appends , when hasMoreMandatoryArgs.
• buildSuggestionsFromHints extended to fall back from the existing entityType dispatch to a hint.kind dispatch. Because this path lives inside tryExclusiveSuggestions, returning items short-circuits the composite path → fields are excluded by construction.
• STATS expectsAggregation opt-out: buildCustomFilteringContext was adding every AGG name to functionsToIgnore (because isAggFunctionUsedAlready saw SPARKLINE itself in the AST). When the cursor's param has hint.kind === 'aggregation', the "no nested aggregations" rule does not apply — nesting is required, not forbidden.

3 production files + 3 caller guards + 1 SPARKLINE unit test + 5 focused unit tests for the new resolver (return-type filter, ignored guard, comma handling, etc.).

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Unit or functional tests were updated or added to match the most common scenarios
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

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