test(engine): cover #108, #58, #125 regression scenarios

Add regression coverage for the discovery-helper changes:

- TestDefaultLinkByGatewayHelpers_MultiNIC: a node with two table=main default
  routes plus a third in an alternate routing table. Asserts that
  default_link_name_by_gateway and default_link_address_by_gateway return a
  single value (the first matching main-table route) and never concatenate.
- TestNetworkMultidoc_NoDiscovery / TestNetworkLegacy_NoDiscovery: render the
  cozystack chart with empty discovery and assert no LinkConfig/BondConfig/
  VLANConfig/Layer2VIPConfig is emitted (multidoc) and no 'interfaces:' or
  empty '- interface:' block appears (legacy).
- TestSecondaryNicHelpers: render a chart that calls each new per-link
  helper against a two-NIC topology (eth0 primary with default route, eth1
  storage with static routes only) and validate every output.

Existing offline tests that asserted broken-empty LinkConfig output
(TestMultiDocCozystack_ControlPlane, TestMultiDocCozystack_LegacyFallback,
TestMultiDocGeneric_ControlPlane, TestMultiDocGeneric_Worker) move from a
no-discovery offline render to renderChartTemplateWithLookup with
simpleNicLookup so they continue to validate the populated LinkConfig path.
TestMultiDocCozystack_BondTopology and _VlanOnBondTopology stay byte-for-byte
identical.

Assisted-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

Author: lexfrei