test: Add comprehensive tests for MultiPathRoute device-only syntax

test: Add comprehensive tests for MultiPathRoute device-only syntax

Add unit tests for config_parse_multipath_route() covering the new
device-only nexthop syntax (@device). Tests verify basic parsing of
device-only routes, gateway with device routes, gateway-only routes,
interface index handling, etc. Also some checks on new hash/duplicate
semantics.

Also add an integration test to verify device-only multipath routes are
correctly installed in the kernel routing table.