[Question]: External tool suspend still emits a synthetic tool result [Awaiting external execution]

## 中文

### 问题

外部工具挂起时，运行时除了发出 `RequireExternalExecutionEvent`，为什么还会自己合成一条工具结果，内容是 `[Awaiting external execution]`？

外部工具此时还没有真正执行。发出 `ToolResultBlock` / `ToolResult*` 事件序列，看起来像工具已经产出了 output，这和 HITL 约定冲突：真正的结果应该由外部回传。

### 当前行为

`ToolExecutor.executeCore` 在可用性与 schema 校验通过后，对 `SchemaOnlyTool` / `@Tool(externalTool=true)` 短路：

```java
if (tool instanceof ToolBase tb && tb.isExternalTool()) {
return Mono.just(ToolResultBlock.suspended(toolCall));
}
```

`ToolResultBlock.suspended()` 把 `[Awaiting external execution]`（或 `ToolSuspendException.getReason()`）写入 `output`，并设置 `metadata.agentscope_suspended = true`。`state` 默认是 `RUNNING`。

`ReActAgent.runToolBatch` 随后把它当成普通工具结果发出：

1. `ToolResultStartEvent`
2. `ToolResultTextDeltaEvent`，内容为 `[Awaiting external execution]`
3. `ToolResultEndEvent(RUNNING)`
4. 然后才是 `RequireExternalExecutionEvent`

`buildSuspendedMsg()` 还会把 `ToolUseBlock` 和这条合成的 `ToolResultBlock` 一起放进返回的 `Msg`（`GenerateReason.TOOL_SUSPENDED`）。

### 为什么这看起来不对

1. **两套信号叠在一起。** 挂起已经由 `RequireExternalExecutionEvent` + `GenerateReason.TOOL_SUSPENDED` 表达。再伪造一条工具结果，等于同时说「这是 output」和「请到外面执行」。
2. **占位文案和真实 output 共用同一字段。** 消费方如果直接渲染 `ToolResultBlock.output` / `ToolResultTextDeltaEvent`，而不检查 `isSuspended()`，就会把 `[Awaiting external execution]` 当成工具成功返回。协议适配器必须特判（AG-UI 已在 `ToolResultEndEvent(RUNNING)` 时丢掉 buffer、不发 `ToolCallResult`；其它适配器未必如此）。
3. **和权限 HITL 不一致。** `RequireUserConfirmEvent` 暂停时不会伪造工具结果。外部执行应该一样：暂停并等待，不要发明 output。
4. **恢复时同一个 `toolCallId` 会出现第二次结果。** 占位结果先被流式发出，调用方稍后注入真正的 `ToolResultBlock`。UI / trace / chat-completions 适配器可能对同一次调用看到两条结果。

### 期望行为（建议）

短路本身没问题：不要调用 `callAsync`，不要注入 preset，不要在本地执行。

需要改的是挂起的表达方式：

- **不要**把占位文案写入 `ToolResultBlock.output`。
- **不要**对尚未执行的工具发出 `ToolResultStart` / `TextDelta` / `End`。
- 只通过 `RequireExternalExecutionEvent` 暴露待执行调用（返回 `Msg` 用 `GenerateReason.TOOL_SUSPENDED`，带 `ToolUseBlock`，不要带假结果）。
- 若 acting 循环仍需要内部标记，放在 metadata 或专用 state 里，不要放进 `output`。

或者，如果必须保留 `ToolResultBlock` 做配对：`output` 留空 + 明确的 `ToolResultState`（例如专用 `SUSPENDED`，不要复用 `RUNNING`）+ 在外部结果到达前跳过 `ToolResult*` 事件。

### 想请维护者确认

1. 这条合成的 `[Awaiting external execution]` 结果，是为了 LLM 上下文 / 协议兼容而有意设计的，还是把挂起编码成 `ToolResultBlock` 时留下的副作用？
2. `runToolBatch` 在 `result.isSuspended()` 时，是否应该跳过 `emitToolResultDelta` / `ToolResultEndEvent`？
3. `ToolResultBlock.suspended()` 是否应该停止把占位文案写入 `output`？

---

## English

### Question

When an external tool is suspended, why does the runtime itself emit a tool result with the placeholder `[Awaiting external execution]`, in addition to `RequireExternalExecutionEvent`?

An external tool has not executed yet. Emitting a `ToolResultBlock` / `ToolResult*` event sequence makes it look like the tool already produced output. That conflicts with the HITL contract: the real result should come from outside the agent.

### Current behavior

`ToolExecutor.executeCore` short-circuits `SchemaOnlyTool` / `@Tool(externalTool=true)` after availability + schema validation:

```java
if (tool instanceof ToolBase tb && tb.isExternalTool()) {
return Mono.just(ToolResultBlock.suspended(toolCall));
}
```

`ToolResultBlock.suspended()` fills `output` with `[Awaiting external execution]` (or `ToolSuspendException.getReason()`), and sets `metadata.agentscope_suspended = true`. `state` defaults to `RUNNING`.

`ReActAgent.runToolBatch` then treats this like a normal tool result:

1. `ToolResultStartEvent`
2. `ToolResultTextDeltaEvent` with `[Awaiting external execution]`
3. `ToolResultEndEvent(RUNNING)`
4. `RequireExternalExecutionEvent`

`buildSuspendedMsg()` also puts both the `ToolUseBlock` and this synthetic `ToolResultBlock` into the returned `Msg` (`GenerateReason.TOOL_SUSPENDED`).

### Why this looks wrong

1. **Two overlapping signals.** Suspend is already represented by `RequireExternalExecutionEvent` + `GenerateReason.TOOL_SUSPENDED`. A fabricated tool result is a second, conflicting signal: "here is the output" vs "please execute this outside".
2. **Placeholder lives in the same field as real output.** Consumers that render `ToolResultBlock.output` / `ToolResultTextDeltaEvent` without checking `isSuspended()` will show `[Awaiting external execution]` as if the tool succeeded. Protocol adapters have to special-case this (AG-UI already drops the buffer on `ToolResultEndEvent(RUNNING)` and does not emit `ToolCallResult`; other adapters may not).
3. **Inconsistent with permission HITL.** `RequireUserConfirmEvent` pauses without synthesizing a tool result. External execution should be the same: pause and wait, do not invent output.
4. **Resume produces a second result for the same `toolCallId`.** The placeholder is streamed first; the caller later injects the real `ToolResultBlock`. UIs / traces / chat-completions adapters can see two results for one call.

### Expected behavior (proposal)

The short-circuit itself is fine: do not invoke `callAsync`, do not inject presets, do not run locally.

What should change is how suspend is represented:

- Do **not** put placeholder text into `ToolResultBlock.output`.
- Do **not** emit `ToolResultStart` / `TextDelta` / `End` for a tool that has not executed.
- Surface the pending call only via `RequireExternalExecutionEvent` (and `GenerateReason.TOOL_SUSPENDED` on the returned `Msg`, with `ToolUseBlock`s but no fake result).
- If an internal marker is still needed for the acting loop, keep it in metadata / a dedicated state, not in `output`.

Alternatively, if a `ToolResultBlock` must exist for pairing: empty `output` + explicit `ToolResultState` (e.g. a dedicated `SUSPENDED`, not reused `RUNNING`) + skip `ToolResult*` events until the external result arrives.

### Questions for maintainers

1. Is the synthetic `[Awaiting external execution]` result intentional for LLM context / protocol compatibility, or leftover from encoding suspend as a `ToolResultBlock`?
2. Should `runToolBatch` skip `emitToolResultDelta` / `ToolResultEndEvent` when `result.isSuspended()`?
3. Should `ToolResultBlock.suspended()` stop writing placeholder text into `output`?

### Environment

- Module: `agentscope-core`
- Related: `ToolExecutor.executeCore`, `ToolResultBlock.suspended()`, `ReActAgent.runToolBatch`, `RequireExternalExecutionEvent`