[Bug]: Dynamic subagent notifications drop FAILED/CANCELLED payloads

[richuV]

Summary

Dynamic/background subagent completion notifications lose authoritative FAILED/CANCELLED terminal information. A failed task is announced to the parent as "has completed" with (no output), while the Harness TaskRecord contains the real failed status and error.

Reproduction

Using AgentScope Java main at e3a412ed2cc944e401da861c8d5e464b967724e9 / 2.0.1-SNAPSHOT:

1. Build a HarnessAgent with dynamic subagents and the default workspace-backed task repository/message bus.
2. Spawn a background child whose invocation terminates with IllegalStateException("failure-marker").
3. Wait until TaskRepository.getTask(...) reports FAILED and contains failure-marker.
4. Continue the parent so InboxMiddleware drains the completion notification.
5. Inspect the injected HintBlock.

Actual hint:

<system-notification>Background subagent task '...' (agent=...) has completed.

Result:

(no output)</system-notification>

The parent never receives the authoritative failed status or error. The same callback shape cannot represent cancellation reason/status either.

Source evidence

HarnessAgent.Builder.wireTaskRepositoryMessageBus(...) installs a WorkspaceTaskRepository completion callback whose arguments end at nullable result. It formats every terminal callback as "has completed" and uses:

result != null ? result : "(no output)"

No task status, error message, or cancellation payload is available to the notification formatter.

Expected behavior

The Harness-owned completion delivery should carry a typed terminal outcome into parent reasoning:

• task id and subagent identity
• terminal status (COMPLETED / FAILED / CANCELLED)
• result for success
• authoritative error for failure
• cancellation reason when available

The fix should remain in the Harness task/message-bus lifecycle. Consumers should not need to add a second completion callback, task store, or delivery registry.

Impact

This blocks reliable parent aggregation and user-visible partial-failure reporting for background collaboration: the task readback says FAILED while automatic parent reasoning says only "(no output)".