Fix pending tasks not being cancelled on client close

[Huguitis]

When a client shuts down, any background tasks that were waiting for the client to be ready would get stuck forever, they'd never unblock because the shutdown wasn't waking them up. Python would then forcefully destroy them and log a flood of "Task was destroyed but it is pending!" warnings

[Huguitis]

Still working on a proper fix for this error before it is merged:

future: <Task finished name='Task-105776' coro=<XMPPClient.event_blocklist_remove() done, defined at /home/container/.local/lib/python3.13/site-packages/rebootpy/xmpp.py:464> exception=RuntimeError('Client is closed.')> Traceback (most recent call last): File "/home/container/.local/lib/python3.13/site-packages/rebootpy/xmpp.py", line 472, in event_blocklist_remove data = await self.client.fetch_user(account_id, raw=True) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/container/.local/lib/python3.13/site-packages/rebootpy/client.py", line 1093, in fetch_user data = await self.fetch_users((user,), cache=cache, raw=raw) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/container/.local/lib/python3.13/site-packages/rebootpy/client.py", line 1182, in fetch_users d = await asyncio.gather(*chunk_tasks) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/container/.local/lib/python3.13/site-packages/rebootpy/http.py", line 1150, in account_get_multiple_by_user_id return await self.get(r, params=params, **kwargs) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/container/.local/lib/python3.13/site-packages/rebootpy/http.py", line 820, in get return await self.fn_request('GET', route, auth, **kwargs) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/container/.local/lib/python3.13/site-packages/rebootpy/http.py", line 645, in fn_request raise RuntimeError('Client is closed.') RuntimeError: Client is closed.

[Huguitis]

I've tested all the changes and they seem to be working correctly.