Function build_favorites_endpoint 

fn build_favorites_endpoint(
    user_id: &str,
    scope: SearchScope,
    options: Option<&GetItemsOptions>,
) -> String

Build the Jellyfin endpoint for a favourites listing.

Pulled out of get_favorites so the query can be asserted without an HTTP server. scope is expanded here — SearchScope::All yields None, and the IncludeItemTypes filter is then omitted entirely rather than sent as a union, which would silently drop every type nobody enumerated (see SearchScope::item_types).

TRACES: UR-067 | DR-115, JA-033 | UT-100