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/README.md

@@ -2,14 +2,16 @@
 
 Plain-Python test suite using stdlib `unittest`. No external
 dependencies. Unit tests run anywhere Python 3 is present; integration
-tests need Docker and skip cleanly otherwise.
+tests run through the backend named by `BOT_BOTTLE_BACKEND` (default
+`docker`) and skip cleanly when that backend isn't available on the host.
 
 ## Layout
 
 ```
 tests/
   fixtures.py                       # JSON manifest builders (shared)
-  _docker.py                        # docker-availability skip helper (shared)
+  _backend.py                       # backend capability probes + skip guards
+  backend_preflight.py              # `python -m tests.backend_preflight` (CI)
   unit/
     test_egress.py
     test_egress_addon_core.py
@@ -73,7 +75,7 @@ BOT_BOTTLE_RUN_CANARIES=1 python -m unittest discover -t . -s tests/canaries -v
 ## Adding a test
 
 1. Pick the directory: `tests/unit/` for a pure unit test,
-   `tests/integration/` for one that needs Docker.
+   `tests/integration/` for one that needs a backend.
 2. Filename: `test_<topic>.py`.
 3. Boilerplate:
    ```python
@@ -88,5 +90,16 @@ BOT_BOTTLE_RUN_CANARIES=1 python -m unittest discover -t . -s tests/canaries -v
    if __name__ == "__main__":
        unittest.main()
    ```
-4. For Docker-dependent tests, decorate the class with
-   `@skip_unless_docker()` from `tests._docker`.
+4. Skip guards live in `tests._backend`:
+   - Backend-agnostic tests (go through `get_bottle_backend()`) decorate
+     the class with `@skip_unless_selected_backend_available()` — the test
+     runs against whichever backend `BOT_BOTTLE_BACKEND` selects and skips
+     unless that backend's capability is present (a reachable Docker daemon,
+     or an accessible `/dev/kvm` + `firecracker` for Firecracker).
+   - Backend-specific tests (exercise `DockerBroker`, `DockerGateway`,
+     `backend.docker.*`, …) decorate with `@skip_unless_backend("docker")`
+     so they no-op under a run targeting a different backend.
+
+   The same capability probes back the CI preflight,
+   `python -m tests.backend_preflight [<backend>]`, which prints a clear
+   PASS/FAIL line and exits non-zero when the selected backend is missing.