Function single_source_bellman_ford_path 

pub fn single_source_bellman_ford_path(
    graph: &Graph,
    source: &str,
    weight_attr: &str,
) -> Option<Vec<(String, Vec<String>)>>

Single-source Bellman-Ford returning paths only. Returns None if a negative cycle is detected. Matches networkx.single_source_bellman_ford_path(G, source, weight=weight).