fix(sandbox): avoid blocking sandbox readiness polling (#2822)

* fix(sandbox): offload async sandbox acquisition

Run blocking sandbox provider acquisition through the async provider hook so eager sandbox setup does not stall the event loop.

* fix(sandbox): add async readiness polling

Introduce an async sandbox readiness poller using httpx and asyncio.sleep while preserving the existing synchronous API.

* test(sandbox): cover async readiness polling

Lock in non-blocking readiness behavior so the async helper does not regress to requests.get or time.sleep.

* fix(sandbox): allow anonymous backend creation

* fix(sandbox): use async readiness in provider acquisition

* fix(sandbox): use async acquisition for lazy tools

* test(sandbox): cover anonymous remote creation

* fix(sandbox): clamp async readiness timeout budget

* fix(sandbox): offload async lock file handling

* fix(sandbox): delegate async middleware fallthrough

* docs(sandbox): document async acquisition path

* fix(sandbox): offload async sandbox release

* docs(sandbox): mention async release hook

* fix(sandbox): address async lock review

Reduce duplicate sync/async sandbox acquisition state handling and move async thread-lock waits onto a dedicated executor with cancellation-safe cleanup.

* chore: retrigger ci

Retrigger GitHub Actions after upstream main fixed the stale PR merge lint failure.

* test(sandbox): sync backend unit fixtures

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.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/)
+- **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.
 - **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