test(transport): add 13 L4 E2E bridge integration tests

Tests exercise DRS/DOM/ORR bridges through the full transport chain:
bridge → NodeTransport → RecordingSender → verify payload bytes.

DRS (4 tests): resolve_and_send delivery, broadcast fan-out, discovery
lookup, unhealthy sender failover.

DOM (4 tests): intent broadcast delivery, DGP header verification,
propagation failure, sequential multi-intent broadcast.

ORR (5 tests): hop forwarding delivery, next_gateway as source, empty
payload, discovery lookup, all-senders-unhealthy error.

Author: mmacedoeu