[ESQL] Adds HeapAttack tests for LIMIT BY and SORT | LIMIT BY

[ncordon]

Adds HeapAttackLimitByIT, covering circuit-breaking inside GroupedLimitOperator and GroupedTopNOperator. Shared helpers are moved to HeapAttackTestCase.

Paths tested:

• GroupedLimitOperator: seenKeys hash table (wide group keys), GroupKeyEncoder scratch buffer (many large strings), large group limit (N in LIMIT N BY)
• GroupedTopNOperator: TopNRow storage (many sort columns), keysHash (wide group keys), TopNQueue pre-allocation (many groups × large per-group limit) and GroupKeyEncoder scratch buffer (many large strings), large group limit (N in LIMIT N BY)

Path not tested: counts array in GroupedLimitOperator: only 4 bytes/group, so seenKeys always trips first.

Also adds assertCircuitBreaksVia(TryCircuitBreaking, String... classNames) which validates the exception stack trace contains the expected classes, and retries if a different part of the pipeline trips first.

Addresses https://github.com/elastic/esql-planning/issues/404

[elasticsearchmachine]

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

[Copilot]

Pull request overview

Adds new ESQL heap-attack REST tests focused on circuit-breaking behavior in LIMIT BY and SORT | LIMIT BY operators, and factors shared heap-attack helpers into HeapAttackTestCase to support these tests.

Changes:

• Add HeapAttackLimitByIT covering multiple circuit-breaking paths in GroupedLimitOperator and GroupedTopNOperator.
• Add assertCircuitBreaksVia(...) plus shared dataset/query helpers to HeapAttackTestCase (and remove duplicates from HeapAttackIT).
• Adjust compute/operator code to support the new tests (resource cleanup + class visibility) and add test module dependencies.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/topn/TopNRow.java	Makes TopNRow public so heap-attack tests can reference it for stack-trace assertions.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/topn/TopNQueue.java	Makes TopNQueue public so heap-attack tests can reference it for stack-trace assertions.
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/GroupedLimitOperator.java	Ensures counts is released on constructor failure.
test/external-modules/esql-heap-attack/src/javaRestTest/java/org/elasticsearch/xpack/esql/heap_attack/HeapAttackTestCase.java	Adds assertCircuitBreaksVia(...) and shared index/query helpers (initManyLongs, makeManyLongs, etc.).
test/external-modules/esql-heap-attack/src/javaRestTest/java/org/elasticsearch/xpack/esql/heap_attack/HeapAttackLimitByIT.java	New integration tests for heap-attack coverage of LIMIT BY and SORT | LIMIT BY.
test/external-modules/esql-heap-attack/src/javaRestTest/java/org/elasticsearch/xpack/esql/heap_attack/HeapAttackIT.java	Removes duplicated helper methods now hosted in HeapAttackTestCase.
test/external-modules/esql-heap-attack/build.gradle	Adds test dependencies on :x-pack:plugin:esql:compute and :libs:swisshash.

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[julian-elastic]

Overall looks good, should be good to merge after the comments are addressed!