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/consolidated_launch.py

@@ -41,8 +41,9 @@ from ...orchestrator.lifecycle import (
 from ...orchestrator.reprovision import reprovision_bottles
 from ...orchestrator.store.secret_store import ENV_VAR_SECRET_NAME
 from ..provision_bottle import deprovision_bottle, provision_bottle
-from . import cleanup, infra_vm, util
+from . import cleanup, util
 from .gateway import FirecrackerGateway
+from .infra import FirecrackerInfraService
 
 _ENV_VAR_SECRET_PATH = "/run/bot-bottle/env-var-secret"
 
@@ -122,13 +123,13 @@ def launch_consolidated(
     """Ensure the infra VM is up, register the bottle by its guest IP, and
     provision its git-gate state into the gateway VM. Returns the context the
     agent-VM launch needs. Raises on failure — the caller tears down."""
-    infra = infra_vm.ensure_running()
-    url = infra.orchestrator_url
+    service = FirecrackerInfraService()
+    url = service.ensure_running()
     client = OrchestratorClient(url)
     _reprovision_running_bottles(client)
 
     # Read the gateway's provisioning transport + CA off the Gateway service.
-    gateway = infra.gateway
+    gateway = service.gateway()
     transport = gateway.provisioning_transport()
     reg = provision_bottle(
         client, guest_ip, egress_plan, git_gate_plan, transport,