fix: Add id field to sort config for consisteny pagination (#421)

**Description:**
The pagination does not work correctly for rel: `next` section in
/search endpoint of the returned product list. This commit adds id
parameter to sorting so that token obtains a unique value.

**PR Checklist:**

- [x] Code is formatted and linted (run `pre-commit run --all-files`)
- [x] Tests pass (run `make test`)
- [ ] Documentation has been updated to reflect changes, if applicable
- [x] Changes are added to the changelog

---------

Co-authored-by: Yuri Zmytrakov <[email protected]>

Author: YuriZmytrakov

CHANGELOG.md

@@ -8,10 +8,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+- Added `id` field as secondary sort to sort config to ensure unique pagination tokens. [#421](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/421)
+- Added default environment variable `STAC_ITEM_LIMIT` to SFEOS for result limiting of returned items and STAC collections [#419](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/419)
+
 ### Changed
 
 - Simplified Patch class and updated patch script creation including adding nest creation for merge patch [#420](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/420)
-- Added default environment variable `STAC_ITEM_LIMIT` to SFEOS for result limiting of returned items and STAC collections [#419](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/419)
 
 ## [v6.2.0] - 2025-08-27
 

stac_fastapi/sfeos_helpers/stac_fastapi/sfeos_helpers/database/query.py

@@ -80,11 +80,14 @@ def populate_sort_shared(sortby: List) -> Optional[Dict[str, Dict[str, str]]]:
         This function transforms a list of sort specifications into the format required by
         Elasticsearch/OpenSearch for sorting query results. The returned dictionary can be
         directly used in search requests.
+        Always includes 'id' as secondary sort to ensure unique pagination tokens.
     """
     if sortby:
-        return {s.field: {"order": s.direction} for s in sortby}
+        sort_config = {s.field: {"order": s.direction} for s in sortby}
+        sort_config.setdefault("id", {"order": "asc"})
+        return sort_config
     else:
-        return None
+        return {"id": {"order": "asc"}}
 
 
 def add_collections_to_body(