fix: keep strong refs to pipeline tasks to prevent GC

[KBVsent]

EventBus dispatched every incoming event with a bare asyncio.create_task(scheduler.execute(event)) whose return value was discarded. Because asyncio only holds weak references to tasks, a suspended pipeline task could be garbage-collected while still pending whenever it lost the reference to the future it was awaiting — for example when an underlying platform/network connection was dropped mid-handler. The result was a recurring Task was destroyed but it is pending! warning followed by a cascade of aclose(): asynchronous generator is already running errors, as the GC tried to finalize the nested suspended async generators of the onion-model pipeline. The work itself usually completed, but the noise masked real failures and exceptions were silently swallowed (Task exception was never retrieved).

Modifications / 改动点

• astrbot/core/event_bus.py: Hold a strong reference to each dispatched pipeline task in a set for the duration of its execution, preventing premature GC. A _on_task_done callback removes the task when finished and logs any uncaught exception via logger.error(..., exc_info=exc), so pipeline failures now surface as a real log entry instead of being lost.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

---

Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Ensure event pipeline tasks remain alive for the duration of their execution so their failures are surfaced instead of being silently discarded.

Bug Fixes:

• Prevent dispatched pipeline tasks from being garbage-collected while still pending by keeping strong references until completion.
• Log uncaught exceptions from pipeline task execution so previously hidden task failures are now visible in logs.

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="astrbot/core/event_bus.py" line_range="63" />
<code_context>
+            return
+        exc = task.exception()
+        if exc is not None:
+            logger.error("pipeline 任务执行异常", exc_info=exc)

     def _print_event(self, event: AstrMessageEvent, conf_name: str) -> None:
</code_context>
<issue_to_address>
**issue (bug_risk):** Passing the exception instance directly to `exc_info` will not log the task traceback as intended.

`exc_info` should be either a truthy flag (to use `sys.exc_info()`) or an `(exc_type, exc, tb)` tuple. Passing the exception instance (`exc_info=exc`) uses the current exception context, which is likely empty here, so the task’s traceback is lost. Instead, build the tuple explicitly so the task stack trace is logged:

```python
exc = task.exception()
if exc is not None:
    logger.error(
        "pipeline 任务执行异常",
        exc_info=(type(exc), exc, exc.__traceback__),
    )
```
</issue_to_address>

---

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[gemini-code-assist]

Code Review

This pull request introduces a mechanism to hold strong references to running asyncio tasks in astrbot/core/event_bus.py to prevent them from being garbage collected while in a pending state. It adds a _pending_tasks set to store these tasks and a _on_task_done callback to discard completed tasks and log any unhandled exceptions. There are no review comments, so I have no feedback to provide.

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