Add configurable sort options to opensearch source search_options

[srikanthpadakanti]

Description

Adds configurable sort option to the opensearch source's search_options, allowing pipeline authors to specify custom sort fields for pagination instead of the hardcoded [_doc asc, _id asc] default.

Example configuration:

source:                                                   
   opensearch:                                                                                                                                     
     search_options:                                                                                                                               
       sort:                                                                                                                                       
         - name: "@timestamp"                                                                                                                      
           order: descending                                                                                                                       
         - name: "_id"                                                                                                                             
           order: ascending                                                                                                                        

When no sort is configured, behavior is unchanged, defaults to _doc ascending + _id ascending for PIT/no-context searches, and _doc ascending for scroll.

Changes include:

• New SortConfig configuration model with name/order fields and validation
• Wired sort options through SearchConfiguration --> workers --> request models --> both OpenSearchAccessor and ElasticsearchAccessor
• Resolved existing TODO comments in PitWorker and NoSearchContextWorker for passing sort options
• Connected the pre-existing but unused SortingOptions model and SearchPointInTimeRequest.sortingOptions field

Issues Resolved

Resolves #6332
#6332

Check List

• [ X ] New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • [ X ] New functionality has javadoc added
• [ X ] Commits are signed with a real name per the DCO

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[dlvenable]

Thanks @srikanthpadakanti ! This looks good, and I only have a few small comments.

[dlvenable]

Using enums helps us with our JSON schema generation and provides a list of valid values. You should be able to search the repo for enums that serve similar purposes and follow the same patterns. WhitespaceOption in the key-value processor is a nice simple example.

You can also add a method to get the OpenSearch name for each.

e.g.

ASCENDING("ascending", "asc"),
DESCENDING("decending", desc");

[srikanthpadakanti]

Good call, updated to use a SortOrder enum following the WhitespaceOption pattern. Also added getSortOrderValue() for the asc/desc mapping.

[dlvenable]

See the enum comment above for a way to handle this in the enum itself.

[srikanthpadakanti]

Addressed.

[dlvenable]

It may be beneficial to detect if _id is present. If it isn't, there is the possibility that we won't be able to break ties. Probably just log a warning for this.

[srikanthpadakanti]

Added a warning log in SortingOptions.fromSortConfigs() when _id is not present in the custom sort configuration.

[srikanthpadakanti]

Thanks @srikanthpadakanti ! This looks good, and I only have a few small comments.

Thank you for the review, @dlvenable. I’ve addressed your comments. Please take a look when you have a moment.

[dlvenable]

Thank you @srikanthpadakanti !