Fix IPv6 route listing on Linux kernels <= 6.9.5

[anigeo]

Summary

Make Linux route listing reliably return IPv6 routes across kernel versions by dumping routes per address family (AF_INET + AF_INET6) and merging results.

Before (old behavior in this crate)

• On Linux <= 6.9.5
  • list() could return IPv4 routes only
  • IPv6 routes missing (None)
• On Linux >= 6.9.6
  • list() typically returns both IPv4 + IPv6 routes (works as expected)

After (this PR)

• On Linux <= 6.9.5
  • list() returns IPv4 + IPv6 routes (fixed)
• On Linux >= 6.9.6
  • list() continues to return IPv4 + IPv6 routes (no regression)

Expected behavior (all kernels)

• RouteManager::list() / AsyncRouteManager::list() should return all current routes (IPv4 + IPv6 when enabled) consistently across kernel versions.

What changed

• Update list_route_req() to accept an AddressFamily and set RouteMessage.header.address_family.
• Update Linux sync + async list() to:
  • dump IPv4 (AddressFamily::Inet)
  • dump IPv6 (AddressFamily::Inet6)
  • merge results
  • return error only if both dumps fail (otherwise return partial results)

Test plan

• Linux <= 6.9.5: confirm IPv6 routes appear in list().
• Linux >= 6.9.6: confirm behavior unchanged.
• Run both sync and async examples (e.g. examples/list.rs, examples/listen_async.rs).