[Stability][BUG-002] Subagent task completes internally but parent task result fails

Parent stability dashboard: #3107

This issue tracks BUG-002 from #3107.

Problem

In Ultra mode, subagents are launched and complete internally, but the parent task tool reports failure instead of returning the subagent result to the lead agent.

Evidence

Source: gateway log, subagent executor + task tool + tool error middleware.

Subagent general-purpose completed async execution
Subagent general-purpose final messages count: ...
Task ... status: completed
Tool execution failed (async): name=task
TypeError: 'AsyncCallbackManager' object is not iterable

Stack root:

deerflow/tools/builtins/task_tool.py
  _report_subagent_usage(runtime, result)
  _find_usage_recorder(runtime)
  for cb in callbacks:
TypeError: 'AsyncCallbackManager' object is not iterable

Source: checkpoint/state inspection of parent tool result.

Error: Tool 'task' failed with TypeError: 'AsyncCallbackManager' object is not iterable. Continue with available context, or choose an alternative tool.

Across multiple Ultra runs:

• subagents were started;
• subagents logged internal completion;
• parent-visible task tool results contained only wrapper errors;
• lead agent fell back to direct work;
• run accounting showed subagent_tokens=0.

Impact

• Ultra mode loses the value of subagent work.
• The lead agent repeats overlapping fetch/research work.
• User-perceived latency and token usage increase.
• Frontend task state can become misleading because the task completed internally but failed at the parent wrapper layer.

Expected behavior

• Completed subagent results should be delivered back to the lead agent.
• Usage-reporting failures should not turn a successful subagent result into a failed task tool result.
• Subagent token accounting should be reflected correctly.