Field collapsing supports search_after

[gaobinlong]

Description

Why field collapsing doesn't support pagination by search_after? The reason is that if we collapse and sort on different field, and use search_after to fetch next page, the same collapsed key may show again in the next page, see the example blow:

Now we collapse on session_id and sort by timestamp ascending, the first page would be:

• s1 - /page1 - 2025-09-07T10:00:00
• s2 - /home - 2025-09-07T10:02:00

, then paging with search_after, the second page would be:

• s2 - /contact - 2025-09-07T10:07:00
• s1 - /page2 - 2025-09-07T10:05:00

, the above behavior would break the field collapsing, but we could imagine that if we collapse and sort on same filed, search_after will work.

So this PR adds the search_after support for field collapsing, but limit the usage that collapse field and sort field must be the same and secondary sort is not supported.

Related Issues

Resolves #3725

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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❌ Gradle check result for 9dc634d: FAILURE

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[gaobinlong]

Hi @reta @andrross , could you help to review this PR, thanks!

[reta]

Hi @reta @andrross , could you help to review this PR, thanks!

Apologies for the delay, @gaobinlong , I will take a look today / tomorrow, thank you

[reta]

Thanks a lot @gaobinlong , a few minor comments but LGTM otherwise, apologies for the delay

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❌ Gradle check result for 62767e6: FAILURE

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✅ Gradle check result for 62767e6: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 77.55102% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.88%. Comparing base (a3ac746) to head (62767e6).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...rg/opensearch/search/collapse/CollapseContext.java	20.00%	2 Missing and 2 partials ⚠️
...main/java/org/opensearch/search/SearchService.java	0.00%	3 Missing ⚠️
...ne/search/grouping/CollapsingTopDocsCollector.java	93.75%	0 Missing and 2 partials ⚠️
...ensearch/search/query/TopDocsCollectorContext.java	77.77%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19261      +/-   ##
============================================
- Coverage     72.89%   72.88%   -0.01%     
- Complexity    69763    69765       +2     
============================================
  Files          5667     5667              
  Lines        320547   320590      +43     
  Branches      46348    46357       +9     
============================================
+ Hits         233648   233654       +6     
- Misses        68005    68038      +33     
- Partials      18894    18898       +4     

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