[ES|QL] Suggest only aggregation functions for SPARKLINE first argument - approach 1

[momovdg]

resolves: #265936

Summary

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

Surfaces the existing getAllFunctions({ type }) filter through the composite-suggestion path, gated on hint.kind:

• ParameterHint.entityType → optional, since SPARKLINE's hint has only kind. Three callers that index parametersFromHintsResolvers[hint.entityType] are guarded.
• New HINT_KIND_SUGGESTION_CONFIG map in empty_expression.ts translates hint.kind → { functionTypes, suppressFields }. Adding a kind = adding one entry; nothing else changes.
• buildFieldAndFunctionSuggestions reads hint.kind and (a) skips addFields when suppressFields is set, (b) passes functionTypes to addFunctions.
• functionTypes threaded through SuggestionBuilder.addFunctions → getFunctionsSuggestions → getAllFunctions, which already supports type filtering.
• 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.

Result for STATS SPARKLINE(|, ...): only AGG/TIME_SERIES_AGG functions are suggested, no fields. Self-recursion is still prevented (SPARKLINE itself stays in the ignore list).

5 production files + 3 caller guards + 1 SPARKLINE-on-STATS unit test.

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[momovdg]

some changes related to entityType are added because this property is now optional (based on the functions definitions).

[momovdg]

this part is a little bit strange because now we are allowing nested aggregations (specific to this case only) compared to the initial requirement