Add missing regression test for manual ip4table netifd classification

[Copilot]

The review pointed out that the PR description claimed a regression test for the netifd/manual-ip4table classification bug, but no such test was present. This change adds that missing coverage without changing production code.

• Regression coverage

  • Adds tests/05_interfaces/04_netifd_mark_chain.
  • Reproduces the case where an interface has a manual ip4table set in UCI but is not pbr-managed via env.netifd_mark.

• What the test asserts

  • The interface is still treated as a normal pbr-managed interface (action == 'create').
  • The interface remains in ifacesTriggers.
  • pbr emits the per-mark nft chain for that interface.
  • Policy rules still goto the generated pbr_mark_<mark> chain.

• Scope

  • Test-only change.
  • No production logic or config handling was modified.

{
  ".name": "vpn",
  "device": "tun0",
  "proto": "openvpn",
  "ip4table": "manual_vpn"
}

let vpn_gw = filter(result?.gateways || [], g => g.name == 'vpn')?.[0];
let vpn_mark = vpn_gw?.mark || '';

['vpn_action_create', vpn_gw?.action == 'create'];
['vpn_in_ifaces_triggers', index(result?.ifacesTriggers || '', 'vpn') >= 0];
['nft_has_vpn_mark_chain',
  index(nft_content, 'add chain inet fw4 pbr_mark_' + vpn_mark) >= 0];