refactor(backend): InfraService ABC + FirecrackerInfraService

Add a backend-neutral `InfraService` ABC (backend/infra_service.py) so all three
backends present the same composer contract: `orchestrator()` / `gateway()`
accessors, `ensure_running(*, startup_timeout) -> str` (the host control-plane
URL), `stop()`, plus concrete `url()` / `is_healthy()` that delegate to the
orchestrator. `DockerInfraService` and `MacosInfraService` now subclass it.

Give firecracker the missing class: `FirecrackerInfraService`
(backend/firecracker/infra.py) wraps the `infra_vm` substrate as the pair
coordinator (adopt-or-boot-both under the singleton flock). `infra_vm` keeps only
the plane-agnostic substrate; its `ensure_running` / `_adopt` / `InfraEndpoint`
move to the class, and the coordinator helpers it now calls cross-module go
public (`singleton_lock` / `expected_version` / `adoptable` /
`record_booted_version`).

Unify the return type on the way: `ensure_running` returns the URL string
everywhere (was `str` for docker, two different `InfraEndpoint`s for
macOS/firecracker), and those `InfraEndpoint`s are deleted — the services are the
source of truth (callers read `gateway().address()` / `orchestrator().url()`).
docker's `url` property becomes the inherited `url()`. backend.py /
consolidated_launch / image_builder + tests updated.

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

tests/unit/test_docker_infra.py

@@ -37,7 +37,7 @@ class TestDockerInfraService(unittest.TestCase):
         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)
+        self.assertEqual("http://127.0.0.1:8099", self.svc.url())
 
     def test_is_healthy_delegates_to_the_orchestrator(self) -> None:
         self.orch.is_healthy.return_value = True