Bug on async Events methods

The `async def prev(self, count=None)` and `async def next(self, count=None)` methods on [client/viscera/events.py](https://github.com/canonical/python-libmaas/blob/0.6.8/maas/client/viscera/events.py) are not using `await` when calling `self._fetch`, which is causing errors when the async iterator tries to continue reading events of a previous/next page.

How to reproduce:
```python
events = await client.events.query()

async for e in events.forwards():
print(e.event_id, e.event_type)
```

The snippet above will fetch some events and then throw an error:
```
...
1902 Performing PXE boot
1906 Loading ephemeral
1907 Image Deployed
1909 Deployed

Traceback (most recent call last):
File "/maas_tests/env/lib/python3.13/site-packages/maas/client/viscera/events.py", line 259, in __anext__
return next(self._current_iter)
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/maas_tests/env/lib/python3.13/site-packages/marimo/_runtime/executor.py", line 115, in execute_cell_async
await eval(cell.body, glbls)
Cell marimo://test-python-libmaas.py#cell=cell-17, line 3, in
async for e in events.forwards():
print(e.event_id, e.event_type)
File "/maas_tests/env/lib/python3.13/site-packages/maas/client/viscera/events.py", line 262, in __anext__
self._current_iter = reversed(self._current)
~~~~~~~~^^^^^^^^^^^^^^^
TypeError: 'coroutine' object is not reversible
```

To fix, we just need to add `await` to the following lines:
- https://github.com/canonical/python-libmaas/blob/0.6.8/maas/client/viscera/events.py#L169
- https://github.com/canonical/python-libmaas/blob/0.6.8/maas/client/viscera/events.py#L180

maas/client/viscera/events.py の、169 行目と 180 行目で参照されている非同期の prev メソッドと next メソッドから始め、提供されている events.forwards() の非同期イテレーション例で問題を再現してください。イベントページをまたいでイテレーションしても、coroutine オブジェクトや報告されている TypeError が生成されなくなれば完了です。

索引モデルが issue の本文から書いたものです。