Function all_simple_paths 

pub fn all_simple_paths(
    graph: &KnowledgeGraph,
    source: &str,
    target: &str,
    max_length: usize,
) -> Vec<Vec<String>>

Find all simple paths between source and target up to max_length edges.

Returns a vec of paths, each path being a vec of node IDs. Limits to at most 50 paths to prevent combinatorial explosion.