Fetches a previously saved contact filter by id. This route is shallow — the workspace is resolved from
the filter itself, so no workspace_id is required in the path. Use the id (or public_id) returned by
POST /api/v2/workspaces/{workspace_id}/contacts/filters with save: true.

Returns the same resource shape as the generate endpoint, with the identity fields populated. The
stable_id is re-derived from the saved filter, so it can be passed straight to
GET /api/v2/workspaces/{workspace_id}/contacts?stable_id=….