fix(state-machine-tests): DEFI-2779: Register non-local subnets from routing table in registry

[mbjorkqvist]

Fix flaky icrc_ledger_suite_integration_golden_state_upgrade_downgrade_test by registering all subnets referenced in the routing table in the registry. Extract add_cup_contents_and_key_record helper to eliminate code duplication.

The golden state tests use create_routing_table (introduced in PR #1530) to route canister IDs outside the local subnet to a non-existent subnet. This ensures that leftover cross-subnet responses in the golden state backup are routed into a remote stream (where they sit harmlessly) rather than triggering a critical error in the stream builder.

This worked until PR #9449 introduced routing table filtering in try_to_populate_network_topology: non-CloudEngine subnets now filter the routing table to only include entries for subnets that have registry records. Since StateMachineBuilder::build() only registered the local subnet, the non-existent subnet's routing entries were silently dropped, leaving cross-subnet responses unroutable and causing mr_stream_builder_response_destination_not_found critical errors in the golden state test.

The test is flaky (rather than always failing) because, depending on the timing of the snapshot, it may or may not contain messages destined for other subnets.

StateMachineBuilder::build() now derives the subnet list from the routing table and registers minimal registry records (DKG transcript, subnet record, key record) for each non-local subnet via register_non_local_subnet. This ensures their routing table entries survive the filtering.

[mbjorkqvist]

Details of a successful run with the test (using a "known-bad" golden state snapshot) can be found here.

[schneiderstefan]

Thank you and sorry for making your test flaky.