Retain shortest edges from multiedges during network import

Instead of assuming key 0 edges exist (which is often not the case), only retain the shortest edges as in osmnx.convert.to_digraph(), independent of the key.

Continuation of #209 #212

Start by tracing the network-import entry point and compare its multiedge handling with osmnx.convert.to_digraph(), particularly the assumption about edge key 0. Confirm the import retains only the shortest edge for each relevant pair regardless of key, then run the existing import or network tests and verify cases without key 0.

Written by the indexing model from the issue text.