Feature: expose a completed subagent task's terminal ActionEvents (or an on-complete callback) for custom delegation tools

Environment

• openhands-sdk / tools 1.44.1 (TaskToolSet / TaskManager path)

Context

We wrap the task tool under a custom delegation primitive (new_task-style) and need the child's STRUCTURED terminal tool calls (the child ends by calling a custom return_to_caller tool whose arguments are the deliverable). Today the only officially visible outcome of TaskManager.start_task(...) is Task.result — the value of get_agent_final_response(...), i.e. the child's final prose/finish message, not its terminal ActionEvents.

_run_task closes the child conversation in a finally via _evict_task, so the events are gone by the time the caller inspects the Task. The only way to capture them is wrapping the private TaskManager._evict_task at instance level before spawning — a private-API dependency we would like to drop.

Ask

Any of:

• keep a terminal-events snapshot on the completed Task (e.g. Task.terminal_events — the last N ActionEvents, or all tool-call events),
• or an optional on_task_complete(task, conversation_state) callback / hook,
• or an officially supported "structured subtask return" channel (e.g. a finish-with-payload equivalent the parent can read from the Task).

Happy to turn this into a PR if there is a preferred shape.

Start with TaskManager.start_task and trace _run_task through _evict_task, comparing the completed Task.result with get_agent_final_response. Review TaskToolSet and the Task model to determine which supported completion shape fits terminal ActionEvents or structured returns. Done means a documented public channel lets a parent inspect the completed subtask’s structured result without wrapping private APIs.