feat: Full test suite and linter - fixed test failures and increased coverage to 61.1%

internal/routing/graph.go

@@ -200,6 +200,10 @@ func BuildGraphFromStations(stations []StationInfo) *Graph {
 // in the same city, as a fallback when direct route search fails.
 // Uses transfer time constants for duration estimation.
 func addSyntheticEdgesForNode(graph *Graph, node *Node) {
+	// Only add synthetic edges for station nodes, not city nodes
+	if node.Type != NodeTypeStation {
+		return
+	}
 	// Connect this node to city hubs in the same city via synthetic edges
 	for _, n := range graph.Nodes() {
 		if n.Type == NodeTypeCity && n.CityCode == node.CityCode {