Fixing sorted indices for GPU built indices

[mayya-sharipova]

• For flush, vectors are now reordered according to sortMap before building the GPU index, ensuring that HNSW graph node ordinals match the sorted document order.
• Merge on the other hand doesn't require explicit sortMap handling since Lucene's MergedVecto utilities apply docMaps internally.
• Enhanced tests with both approximate and exact KNN searches to validate sorting correctness.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @mayya-sharipova, I've created a changelog YAML for you.

[ldematte]

Change looks good to me; I'm not a Lucene expert so I cannot say if it's the right way to do it so I'll trust you/Chris on this.
You probably want to merge in changes from #138155 and add the test-gpu flag to be sure tests pass/are OK.

[ChrisHegarty]

The changes look good to me. I guess I'm surprised to not see some changes to ES92GpuHnswVectorsFormatTests! Is there no sorting tests there?

[mayya-sharipova]

The changes look good to me. I guess I'm surprised to not see some changes to ES92GpuHnswVectorsFormatTests! Is there no sorting tests there?

These tests use tests inside BaseKnnVectorsFormatTestCase, such as testSortedIndex but testSortedIndex doesn't exercise knn search that use graphs, only goes through vector values.

@ChrisHegarty Do you suggest we need to add tests into ES92GpuHnswVectorsFormatTests, I can do that

[ChrisHegarty]

First, I think that this PR is good to be merged as-is.

These tests use tests inside BaseKnnVectorsFormatTestCase, such as testSortedIndex but testSortedIndex doesn't exercise knn search that use graphs, only goes through vector values.

Right. It surprises me that sorting was completely unimplemented, and that no scenarios in BaseKnnVectorsFormatTestCase needed to be bypassed (and then re-enabled) or something. This seems like a gap in BaseKnnVectorsFormatTestCase, no ?

[mayya-sharipova]

Thanks Chris, I will merge this PR and look into adding more sorted index test into BaseKnnVectorsFormatTestCase

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	9.2