ci: backend-agnostic integration guards + per-backend preflight

Integration tests now select their backend from BOT_BOTTLE_BACKEND and
skip on the capability that backend actually needs, instead of gating
every backend on unrelated Docker availability.

Task 1 — backend-agnostic guards (tests/_backend.py):
- Capability probes: docker_capability() (reachable daemon) and
  firecracker_capability() (accessible /dev/kvm + firecracker on PATH,
  Docker-independent). backend_capability()/selected_backend() resolve
  the target from BOT_BOTTLE_BACKEND (default docker).
- skip_unless_selected_backend_available() for backend-agnostic tests
  (test_sandbox_escape) — runs through whichever backend is selected and
  checks that backend's real capability.
- skip_unless_backend("docker") for Docker-implementation tests
  (DockerBroker, DockerGateway, backend.docker.*) — they no-op under a
  non-Docker run rather than testing internals that run doesn't target.
- Retires tests/_docker.py; the KVM job no longer needs SKIP_DOCKER_TESTS
  to steer Docker-only classes.

Task 2 — explicit per-backend skip visibility:
- tests/backend_preflight.py prints a clear PASS/FAIL capability line and
  exits non-zero when the selected backend is missing.
- Both integration jobs run it as a preflight, so absent infrastructure
  is surfaced at the job level instead of hidden among unittest.skip
  lines. The docker job replaces its soft "Show environment" step; the
  firecracker job keeps its richer backend-status check.

Docs (tests/README.md, docs/ci.md) updated; unit coverage for the probes,
guards, and preflight in test_backend_skip_guards.py.

Closes #414

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

tests/integration/test_gateway_image.py

@@ -25,14 +25,14 @@ import os
 import subprocess
 import unittest
 
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 
 _IMAGE = "bot-bottle-gateway-test:chunk1"
 _DOCKERFILE = "Dockerfile.gateway"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: multi-stage build pulls a 200+MB "

tests/integration/test_multitenant_isolation.py

@@ -31,7 +31,7 @@ from bot_bottle.backend.docker.gateway_net import next_free_ip
 from bot_bottle.orchestrator.client import OrchestratorClient
 from bot_bottle.orchestrator.gateway import GATEWAY_IMAGE, GATEWAY_NAME, GATEWAY_NETWORK
 from bot_bottle.orchestrator.lifecycle import OrchestratorService
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 # One upstream reachable under two names; reflects the Authorization header so
 # the probe can see exactly what the gateway injected (or didn't).
@@ -72,7 +72,7 @@ _PROBE_SRC = (
 )
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: the orchestrator container bind-mounts the repo "

tests/integration/test_orchestrator_docker_broker.py

@@ -13,12 +13,12 @@ import unittest
 
 from bot_bottle.orchestrator.broker import LaunchRequest, sign_request
 from bot_bottle.orchestrator.docker_broker import DockerBroker, container_name
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerBrokerIntegration(unittest.TestCase):
     def setUp(self) -> None:
         self.secret = secrets.token_bytes(16)

tests/integration/test_orchestrator_docker_control_plane_auth.py

@@ -28,7 +28,7 @@ from pathlib import Path
 from bot_bottle.orchestrator.client import OrchestratorClient
 from bot_bottle.orchestrator.lifecycle import OrchestratorService
 from bot_bottle.paths import host_control_plane_token
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 # Fixed (not per-run-suffixed) so repeated runs reuse the same layer-cached
 # image instead of leaking a new dangling tag on every invocation.
@@ -37,7 +37,7 @@ _TEST_GATEWAY_IMAGE = "bot-bottle-gateway:itest"
 _TEST_INFRA_IMAGE = "bot-bottle-infra:itest"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: the orchestrator container bind-mounts the repo "

tests/integration/test_orchestrator_docker_gateway.py

@@ -11,12 +11,12 @@ import subprocess
 import unittest
 
 from bot_bottle.orchestrator.gateway import DockerGateway
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerGatewayIntegration(unittest.TestCase):
     def setUp(self) -> None:
         self.name = "bot-bottle-orch-gateway-itest-" + secrets.token_hex(4)

tests/integration/test_orchestrator_docker_gateway_build.py

@@ -12,12 +12,12 @@ import subprocess
 import unittest
 
 from bot_bottle.orchestrator.gateway import DockerGateway
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerGatewayImageExists(unittest.TestCase):
     def test_image_exists_true_for_present_false_for_absent(self) -> None:
         # Ensure the tiny image is present (build_if_missing is disabled here

tests/integration/test_orphan_cleanup.py

@@ -18,10 +18,10 @@ from bot_bottle.backend.docker.network import (
     network_create_internal,
     network_remove,
 )
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestOrphanCleanup(unittest.TestCase):
     def setUp(self):
         self.slug = f"cb-test-orphan-{os.getpid()}"

tests/integration/test_sandbox_escape.py

@@ -31,7 +31,7 @@ from pathlib import Path
 from bot_bottle.backend import BottleSpec, get_bottle_backend
 from bot_bottle.bottle_state import cleanup_state
 from bot_bottle.manifest import ManifestIndex
-from tests._docker import skip_unless_docker_or_firecracker
+from tests._backend import skip_unless_selected_backend_available
 
 
 # Secrets planted in the bottle env as literals (agents substitute via
@@ -67,7 +67,7 @@ _DUMMY_HOST_KEY = (
 )
 
 
-@skip_unless_docker_or_firecracker()
+@skip_unless_selected_backend_available()
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true"
     and os.environ.get("BOT_BOTTLE_BACKEND") != "firecracker",