Function build_get_items_endpoint 

fn build_get_items_endpoint(
    user_id: &str,
    parent_id: &str,
    options: Option<&GetItemsOptions>,
) -> String

Build the Jellyfin endpoint for a folder listing.

Extracted from get_items so the query it produces — in particular the favourites filter — can be asserted without standing up an HTTP server.

TRACES: UR-007, UR-067 | DR-116 | UT-104