Fix tracked routes being collapsed by incomplete comparison (#41393)

Mirrored route tracking used an incomplete comparator that considered only route class, destination address, and metric. Distinct routes with different prefix lengths or next hops could therefore be treated as equivalent and silently omitted.

The fix preserves the existing route-class ordering while using the complete EndpointRoute comparison for route identity. Regression tests cover prefix length, next hop, metric, exact duplicates, and dependency ordering.