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/firecracker/image_builder.py

@@ -26,7 +26,8 @@ from pathlib import Path
 from typing import Generator
 
 from ...log import die, info
-from . import infra_vm, util
+from . import util
+from .infra import FirecrackerInfraService
 
 # vfs + chroot: buildah works as root in the microVM (no fuse-overlayfs /
 # overlay module / subuid maps). `--isolation` is a build/run-only flag;
@@ -125,8 +126,9 @@ def _build_in_infra(
     orchestrator VM (the build-capable control plane), smoke test the image,
     and stream its rootfs into `base`. The infra VMs persist; only the
     per-build container/image/context are cleaned up."""
-    orchestrator = infra_vm.ensure_running().orchestrator
-    key, ip = orchestrator.ssh_target()
+    service = FirecrackerInfraService()
+    service.ensure_running()
+    key, ip = service.orchestrator().ssh_target()
     tag = f"bot-bottle-agent-build-{digest}"
     ctx = f"/tmp/agent-build-{digest}"
     smoke_ctr, export_ctr = f"{tag}-smoke", f"{tag}-export"