refactor(core): add type hints to pipeline scheduler

[PersonalViolet]

在astrbot\core\pipeline\scheduler.py中为self.stages添加类型标注，让 IDE 与类型检查器能正确推断list[Stage]类型，纯类型层面改动，无功能变化。

Modifications / 改动点

• astrbot/core/pipeline/scheduler.py
  • 将 self.stages 标注为 list[Stage]
  • 在 _process_stages 中，使用 cast(AsyncGenerator[None], coroutine) 来收窄 Stage.process() 返回的生成器类型
  • 导入 Stage 和 cast

• 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

uv run pytest tests/test_smoke.py -v

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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

Refine the pipeline scheduler’s stage handling to use explicit typing for stage instances and their async generators.

Enhancements:

• Annotate the scheduler’s stages collection with a concrete list[Stage] type to improve IDE support and static checking.
• Constrain the type of async generators returned from Stage.process() using typing.cast for more precise type inference.

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The cast(AsyncGenerator[None], coroutine) line is currently using AsyncGenerator from collections.abc, which is not subscriptable; consider importing AsyncGenerator from typing (with proper type parameters) or adjusting the cast to a compatible generic type so type checkers don't error on this annotation.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `cast(AsyncGenerator[None], coroutine)` line is currently using `AsyncGenerator` from `collections.abc`, which is not subscriptable; consider importing `AsyncGenerator` from `typing` (with proper type parameters) or adjusting the cast to a compatible generic type so type checkers don't error on this annotation.

## Individual Comments

### Comment 1
<location path="astrbot/core/pipeline/scheduler.py" line_range="53-54" />
<code_context>
             if isinstance(coroutine, AsyncGenerator):
                 # 如果返回的是异步生成器, 实现洋葱模型的核心
-                async for _ in coroutine:
+                agen = cast(AsyncGenerator[None], coroutine)
+                async for _ in agen:
                     # 此处是前置处理完成后的暂停点(yield), 下面开始执行后续阶段
                     if event.is_stopped():
</code_context>
<issue_to_address>
**issue (bug_risk):** Fix the AsyncGenerator type parameters in cast to avoid runtime/type-checking issues.

`collections.abc.AsyncGenerator` requires two type parameters `(YieldType, SendType)`, so `AsyncGenerator[None]` is invalid and can raise `TypeError` at runtime as well as fail static checks. Use something like `cast(AsyncGenerator[Any, Any], coroutine)` (or `AsyncGenerator[object, object]` if that’s more accurate), or drop the cast and rely on the `isinstance(..., AsyncGenerator)` check if your type checker supports that narrowing.
</issue_to_address>

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[Copilot]

Pull request overview

This PR aims to improve static typing for the pipeline scheduler by explicitly annotating PipelineScheduler.stages as list[Stage] and narrowing the inferred type of the async generator returned by Stage.process().

Changes:

• Add Stage typing for self.stages to improve IDE/type-checker inference.
• Introduce a typing.cast in _process_stages to narrow the async generator type returned from Stage.process().
• Update imports to include cast and Stage.

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