feat: allow sorting endpoints by active findings count

[valentijnscholten]

Summary

• Add ability to sort the endpoint list and host list by active findings count in the UI (ascending and descending)
• Annotate the endpoint queryset with active_finding_count before filtering so the sort operates at the database level
• Add active_finding_count to EndpointFilterHelper and ApiEndpointFilter ordering fields, and expose it in EndpointSerializer
• V3 Locations path: add active_findings ordering to URLFilter (UI) and V3EndpointCompatibleFilterSet (API compat); expose active_finding_count in V3EndpointCompatibleSerializer; the annotation is already provided by annotate_location_counts_and_status and annotate_host_contents
• Update endpoints.html and url/list.html templates to render the column header as a sortable link and use the annotated count directly from the queryset
• Add Selenium integration tests to endpoint_extended_test.py covering ascending and descending sort for both endpoint list and host list views; tests automatically select the correct sort field name based on DD_V3_FEATURE_LOCATIONS

Closes #10540