feat: add task stream_callback

[fengju0213]

Description

Although the previous task supported streamed decomposition, it only allowed incremental output when multiple subtasks were being generated.
To obtain the original streaming content produced by the task-decomposition agent itself, you need to handle the original stream via a callback mechanism — so that external components can directly access and process the raw data in real time.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch task_stream_callback

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[a7m-1st]

LGTM thanks @fengju0213

[waleedalzarooni]

LGTM!

[coolbeevip]

LGTM