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/tests/test_remote_sandbox_backend.py

@@ -159,6 +159,26 @@ def test_provisioner_create_returns_sandbox_info(monkeypatch):
     assert info.sandbox_url == "http://k3s:31001"
 
 
+def test_provisioner_create_accepts_anonymous_thread_id(monkeypatch):
+    backend = RemoteSandboxBackend("http://provisioner:8002")
+
+    def mock_post(url: str, json: dict, timeout: int):
+        assert url == "http://provisioner:8002/api/sandboxes"
+        assert json == {
+            "sandbox_id": "anon123",
+            "thread_id": None,
+            "user_id": "test-user-autouse",
+        }
+        assert timeout == 30
+        return _StubResponse(payload={"sandbox_id": "anon123", "sandbox_url": "http://k3s:31002"})
+
+    monkeypatch.setattr(requests, "post", mock_post)
+
+    info = backend.create(None, "anon123")
+    assert info.sandbox_id == "anon123"
+    assert info.sandbox_url == "http://k3s:31002"
+
+
 def test_provisioner_create_raises_runtime_error_on_request_exception(monkeypatch):
     backend = RemoteSandboxBackend("http://provisioner:8002")