[ESQL] Fix TopN grouping aggregates bug with non-qualifying intermediate states

[ivancea]

Continuation of #127148

When datanodes send the STATS intermediate states to the coordinator, it aggregates them.
Now, however, the TopN groups sent by a datanode may not be acceptable in the coordinator (Because it has better values already), so it will discard such values.

However, the engine wasn't handling intermediate groups with nulls (TopNBlockHash uses nulls to discard unused groups).
See https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/GroupingAggregator.java#L47

This code isn't connected with the query yet, so there's no bug in production

[elasticsearchmachine]

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

[Copilot]

Pull Request Overview

This PR fixes a bug in the TopN grouping aggregates where intermediate states with nulls were mishandled, ensuring that outdated TopN sets received from datanodes are correctly discarded by the coordinator.

• Added new intermediate input methods to aggregation functions.
• Updated tests to cover scenarios where discarding of non-qualifying TopN sets is required.
• Refactored code and generator logic for improved consistency in handling different types of group id blocks.

Reviewed Changes

Copilot reviewed 9 out of 87 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/HashAggregationOperatorTests.java	Added testTopNNullsIntermediateDiscards to validate proper discarding of outdated TopN states.
x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/aggregation/GroupingAggregatorFunctionTestCase.java	Updated to include and test new intermediate input methods, with minor internal assertion changes.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/ToPartialGroupingAggregatorFunction.java	Added method overrides for addIntermediateInput using IntArrayBlock and IntBigArrayBlock.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/GroupingAggregatorFunction.java	Introduced new method signatures for intermediate input handling.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/FromPartialGroupingAggregatorFunction.java	Added intermediate input delegation to the underlying aggregator.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/FilteredGroupingAggregatorFunction.java	Forwarded intermediate input calls to the next aggregator in the chain.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/CountGroupingAggregatorFunction.java	Removed redundant Math.toIntExact conversions and added intermediate input implementations.
x-pack/plugin/esql/compute/gen/src/main/java/org/elasticsearch/compute/gen/GroupingAggregatorImplementer.java	Updated code generator to produce appropriate addIntermediateInput methods based on the group id type.

[Copilot]

[nitpick] Consider replacing the plain 'assert' statement with a proper JUnit assertion (e.g., assertEquals(1, valueCount)) to provide a clearer error message if this assumption fails.

Suggested change

	assert valueCount == 1; // Multi-values make chunking more complex, and it's not a real case yet
	assertEquals(1, valueCount, "Multi-values make chunking more complex, and it's not a real case yet");