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>

bot_bottle/backend/macos_container/backend.py

@@ -106,7 +106,7 @@ class MacosContainerBottleBackend(
         (`supervise`) call when no control plane is running yet. Mirrors
         firecracker's infra bring-up."""
         from .infra import MacosInfraService
-        return MacosInfraService().ensure_running().orchestrator_url
+        return MacosInfraService().ensure_running()
 
     def prepare_cleanup(self) -> MacosContainerBottleCleanupPlan:
         return _cleanup.prepare_cleanup()