树搜索: 单个失败的工作节点会中止整个运行(在 `ParallelAgent.step` 的结果收集循环中调用 `raise`)

> Related issues: RELATED #65 (closed) — that report shows the same failure class (a worker's `JSONDecodeError` propagating through `future.result()` and killing the whole run), but only as a symptom/question. This report identifies the explicit catch-and-re-raise in the result-collection loop as the root cause and proposes a fix. No open duplicate found.