[codex] Fix stale AIO sandbox cache reuse (#3494)

* Fix stale AIO sandbox cache reuse

* Address AIO sandbox review feedback

* Distinguish sandbox health check failures

* Keep local discovery recoverable when the runtime check fails

LocalContainerBackend.discover() shares _is_container_running, which now
raises on transient daemon errors instead of returning False. Discovery has
no exception handling in _discover_or_create_with_lock(_async), so a brief
Docker hiccup turned a recoverable "could not verify, create instead" into a
hard acquire failure. Catch the check failure inside discover() and return
None so an unverifiable container is simply not adopted, restoring the
pre-change fall-through while keeping raise-on-unknown semantics protecting
the destroy path.

Reported by fancy-agent on PR #3494.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Narrow the not-found match in container inspect error handling

A bare "not found" substring also matches transient failures like "command
not found" or "context not found", which would misclassify a check error as
"container definitely gone" and bypass the raise-on-unknown contract. Keep
Docker's specific "No such object"/"No such container" phrases, and only
trust a generic "not found" (Apple Container) when the message names the
inspected container or refers to a container/object.

Reported by WillemJiang on PR #3494.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

backend/README.md

@@ -69,7 +69,7 @@ Middlewares execute in strict order, each handling a specific concern:
 Per-thread isolated execution with virtual path translation:
 
 - **Abstract interface**: `execute_command`, `read_file`, `write_file`, `list_dir`
-- **Providers**: `LocalSandboxProvider` (filesystem) and `AioSandboxProvider` (Docker, in community/). Async runtime paths use async sandbox lifecycle hooks so startup, readiness polling, and release do not block the event loop.
+- **Providers**: `LocalSandboxProvider` (filesystem) and `AioSandboxProvider` (Docker, in community/). Async runtime paths use async sandbox lifecycle hooks so startup, readiness polling, and release do not block the event loop. `AioSandboxProvider` validates active-cache and warm-pool containers during acquire/reuse, dropping definitively dead entries so a thread can provision a fresh sandbox after an unexpected container exit while keeping `get()` as an in-memory lookup. Backend health-check failures are treated as unknown, not dead, and a container that cannot be verified during discovery is simply not adopted (acquire falls through to create instead of failing).
 - **Virtual paths**: `/mnt/user-data/{workspace,uploads,outputs}` → thread-specific physical directories
 - **Skills path**: `/mnt/skills` → `deer-flow/skills/` directory
 - **Skills loading**: Recursively discovers nested `SKILL.md` files under `skills/{public,custom}` and preserves nested container paths