fix(sandbox): add startup reconciliation to prevent orphaned container leaks (#1976)

* fix(sandbox): add startup reconciliation to prevent orphaned container leaks

Sandbox containers were never cleaned up when the managing process restarted,
because all lifecycle tracking lived in in-memory dictionaries. This adds
startup reconciliation that enumerates running containers via `docker ps` and
either destroys orphans (age > idle_timeout) or adopts them into the warm pool.

Closes #1972

* fix(sandbox): address Copilot review — adopt-all strategy, improved error handling

- Reconciliation now adopts all containers into warm pool unconditionally,
  letting the idle checker decide cleanup. Avoids destroying containers
  that another concurrent process may still be using.
- list_running() logs stderr on docker ps failure and catches
  FileNotFoundError/OSError.
- Signal handler test restores SIGTERM/SIGINT in addition to SIGHUP.
- E2E test docstring corrected to match actual coverage scope.

* fix(sandbox): address maintainer review — batch inspect, lock tightening, import hygiene

- _reconcile_orphans(): merge check-and-insert into a single lock acquisition
  per container to eliminate the TOCTOU window.
- list_running(): batch the per-container docker inspect into a single call.
  Total subprocess calls drop from 2N+1 to 2 (one ps + one batch inspect).
  Parse port and created_at from the inspect JSON payload.
- Extract _parse_docker_timestamp() and _extract_host_port() as module-level
  pure helpers and test them directly.
- Move datetime/json imports to module top level.
- _make_provider_for_reconciliation(): document the __new__ bypass and the
  lockstep coupling to AioSandboxProvider.__init__.
- Add assertion that list_running() makes exactly ONE inspect call.

backend/packages/harness/deerflow/community/aio_sandbox/backend.py

@@ -96,3 +96,19 @@ class SandboxBackend(ABC):
             SandboxInfo if found and healthy, None otherwise.
         """
         ...
+
+    def list_running(self) -> list[SandboxInfo]:
+        """Enumerate all running sandboxes managed by this backend.
+
+        Used for startup reconciliation: when the process restarts, it needs
+        to discover containers started by previous processes so they can be
+        adopted into the warm pool or destroyed if idle too long.
+
+        The default implementation returns an empty list, which is correct
+        for backends that don't manage local containers (e.g., RemoteSandboxBackend
+        delegates lifecycle to the provisioner which handles its own cleanup).
+
+        Returns:
+            A list of SandboxInfo for all currently running sandboxes.
+        """
+        return []