Function build_next_up_endpoint 

fn build_next_up_endpoint(
    user_id: &str,
    series_id: Option<&str>,
    limit: Option<usize>,
) -> String

Build the Jellyfin endpoint for a Next Up listing.

EnableResumable=false is the point of this query: the server default is true, which makes a partially-watched episode its own series’ “next up” — the very episode /Items/Resume returns — so Continue Watching and Next Up end up showing the same cards. Next Up should only ever offer episodes the viewer has not started. Servers predating the parameter ignore it, which is why the frontend also drops in-progress entries (DR-197).

Pulled out of get_next_up_episodes so the query can be asserted without an HTTP server, matching build_favorites_endpoint.

TRACES: UR-023, UR-059 | DR-197, JA-014, JA-036 | UT-190, UT-191