chore(backend): drop dead code left by the InfraService refactor

Remove symbols with no remaining consumer:

- `gateway_name` property on Docker/MacosInfraService — nothing uses it; the
  launch flow reads `gateway().name` off the Gateway service. (+ its docker test.)
- firecracker `infra_vm` EGRESS_PORT / SUPERVISE_PORT / GIT_HTTP_PORT — dead
  duplicates; launch.py uses supervisor.types / docker.egress / a local const.
- macOS `INFRA_DB_VOLUME` back-compat alias — unused since the DB-volume test
  moved to `ORCHESTRATOR_DB_VOLUME`.
- docker `infra.py`: the unused `ORCHESTRATOR_SOURCE_HASH_LABEL` import and the
  `__all__` re-exports of the orchestrator constants (consumers import them from
  `docker.orchestrator`, not `docker.infra`).
- macOS `infra.py`: the unused `GatewayError` import + re-export.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_docker_infra.py

@@ -33,9 +33,6 @@ class TestDockerInfraService(unittest.TestCase):
             p.start()
             self.addCleanup(p.stop)
 
-    def test_gateway_name(self) -> None:
-        self.assertEqual(GATEWAY_NAME, self.svc.gateway_name)
-
     def test_url_delegates_to_the_orchestrator(self) -> None:
         self.assertEqual("http://127.0.0.1:8099", self.svc.url())