fix: ensure datetime in post request is normalized

[YuriZmytrakov]

Description:

Currently, in POST requests to /search, with datetime values are not normalized as they are in GET requests to /search, which can cause different results to be returned for the same datetime filter. For example:
POST request to /search with datetime=2025-11-20T09:54:38Z/2025-11-20T09:54:38Z returns an item with 2025-11-20T09:54:38.196614Z, whereas a GET request with the same datetime filter does not which is expected behavior. This PR ensures that datetime values in POST requests are normalized in the same way as in GET requests.

PR Checklist:

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