fix: capture agent end_time before emitting agent_stats

[wcqqq1214]

Modifications / 改动点

• In tool_loop_agent_runner.py, capture end_time right before the agent_stats snapshot is serialized via to_dict(). Previously end_time was only set after the yield (inside _complete_with_assistant_response), so the frontend received end_time=0 and the per-message elapsed duration (耗时) in the webchat detail panel always displayed as "-". Token counts and time-to-first-token were unaffected.

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

Verification Steps / 验证步骤

1. Start AstrBot and open the webchat.
2. Send a message and wait for the reply.
3. Click the detail (info) button on the assistant message and check the 耗时 (duration) row.

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

Before / 修复前

After / 修复后

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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。（本 PR 无新功能）

• 👀 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"。（截图待补充）

• 🤓 I have ensured that no new dependencies are introduced.
  / 我确保没有引入新依赖库。

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

Summary by Sourcery

Bug Fixes:

• Fix agent_stats snapshots reporting an end_time of 0, which caused the frontend message duration to display as '-' instead of the actual elapsed time.

Summary by Sourcery

Bug Fixes:

• Fix agent_stats responses reporting an end_time of 0 that caused the webchat message duration field to show '-' instead of the elapsed time.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

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