fix: ensure isformat does not truncate 0Z (#535)

**Description:**

This PR fixes `.0Z` milliseconds being truncated by `isformat`, causing
items outside the expected datetime range to be returned. The
apply_filter_datetime function was updated to preserve milliseconds when
filtering, ensuring that only items exactly within the specified
datetime range are included.

Example `2023-01-09T13:39:39.0Z` returned item
`2023-01-09T13:39:39.024Z` which is not within the specified range, but
was returned because .0Z was truncated to 2023-01-09T13:39:39Z.

After the fix, queries respect millisecond precision, and only items
strictly within the specified range are returned.

**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 <yzmytrakovNB@yzmytrakovNB.local>

Author: YuriZmytrakov

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 
+- Fixed datetime filtering for .0Z milliseconds to preserve precision in apply_filter_datetime, ensuring only items exactly within the specified range are returned. [#535](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/535)
+
 ### Removed
 
 ## [v6.7.5] - 2025-11-25

stac_fastapi/core/stac_fastapi/core/datetime_utils.py

@@ -32,7 +32,10 @@ def normalize(dt):
                 dt_utc = MIN_DATE_NANOS
             if dt_utc > MAX_DATE_NANOS:
                 dt_utc = MAX_DATE_NANOS
-            return dt_utc.isoformat(timespec="auto").replace("+00:00", "Z")
+            dt_normalized = dt_utc.isoformat(timespec="auto").replace("+00:00", "Z")
+            if "." not in dt_normalized:
+                dt_normalized = dt_normalized.replace("Z", ".0Z")
+            return dt_normalized
 
         if not isinstance(date_str, str):
             return f"{MIN_DATE_NANOS.isoformat(timespec='auto').replace('+00:00','Z')}/{MAX_DATE_NANOS.isoformat(timespec='auto').replace('+00:00','Z')}"