Prevent auto-sharding for data streams in LOOKUP index mode

[lukewhiting]

This PR disabled the datastream autosharding for LOOKUP indices to prevent them scaling above 1 replica which is unsupported by the lookup mappers.

Fixes ES-12330

[elasticsearchmachine]

Hi @lukewhiting, I've created a changelog YAML for you.

[Copilot]

Pull Request Overview

This PR prevents auto-sharding functionality for data streams that use LOOKUP index mode, as lookup mappers don't support scaling beyond 1 replica. The implementation adds an early return in the auto-sharding calculation logic when the index mode is LOOKUP.

• Adds a check in the auto-sharding service to return NOT_APPLICABLE_RESULT for LOOKUP index mode data streams
• Includes comprehensive test coverage for both scenarios with and without index statistics

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
DataStreamAutoShardingService.java	Adds LOOKUP index mode check to prevent auto-sharding calculation
DataStreamAutoShardingServiceTests.java	Adds test cases to verify auto-sharding is disabled for LOOKUP index mode

Comments suppressed due to low confidence (2)

server/src/test/java/org/elasticsearch/action/datastreams/autosharding/DataStreamAutoShardingServiceTests.java:1347

• [nitpick] The test method name could be more descriptive. Consider renaming to 'testCalculateReturnsNotApplicableForLookupIndexModeWithStats' to better distinguish it from the null stats test case.

    public void testCalculateReturnsNotApplicableForLookupIndexMode() {

[Copilot]

There's code duplication between the two test methods. Consider extracting the common DataStream and ClusterState setup into a helper method to improve maintainability.