util: cleanup asyncio event loop after stopping

I noticed many ResourceWarning when running regtests with
PYTHONASYNCIODEBUG=1 and PYTHONDEVMODE=1, each time a daemon
gets stopped the asyncio loop wouldn't get properly cleaned up:

```
(env) user@hp:~/code/electrum-fork$ python3 -m unittest tests.regtest.TestLightningAB.test_lnwatcher_waits_until_fees_go_down
***** test_lnwatcher_waits_until_fees_go_down ******
initializing alice
  0.67 | W | asyncio | Executing <Task finished name='Task-1' coro=<run_offline_command() done, defined at /home/user/code/electrum-fork/./run_electrum:229> result={'msg': 'Please keep ... your wallet.', 'path': '/tmp/alice/r...efault_wallet', 'seed': 'fiction sadd...it radar desk'} created at /home/user/code/electrum-fork/electrum/util.py:1760> took 0.280 seconds
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
funding alice
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
f84277454a04243e500cf84c67aad16e04dd7a88ffa849ffcf20ce3f9af277df
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
initializing bob
  0.54 | W | asyncio | Executing <Task finished name='Task-1' coro=<run_offline_command() done, defined at /home/user/code/electrum-fork/./run_electrum:229> result={'msg': 'Please keep ... your wallet.', 'path': '/tmp/bob/reg...efault_wallet', 'seed': 'wink loud so...ory myth case'} created at /home/user/code/electrum-fork/electrum/util.py:1760> took 0.195 seconds
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
funding bob
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
f68b651e84dc8547f54dd09129018a2d0d256dedc8ccc48595ae172de895371a
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
mining 1 blocks
starting daemon (PID 38153)
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
/tmp/alice/regtest/wallets/default_wallet
/usr/lib64/python3.14/asyncio/base_events.py:758: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
```

This commits adds some cleanup to `util.create_and_start_event_loop()`
to
1. cancel remaining tasks
2. shut down asyncgens
3. shutdown the default executor
4. call loop.close() to free the resources allocated to the loop
See https://stackoverflow.com/questions/30765606/whats-the-correct-way-to-clean-up-after-an-interrupted-event-loop

This seems to reliably solve the mentioned `ResourceWarning`.

Author: f321x

electrum/util.py

@@ -52,7 +52,7 @@
 from functools import partial
 from abc import abstractmethod, ABC
 import enum
-from contextlib import nullcontext
+from contextlib import nullcontext, suppress
 import traceback
 import inspect
 
@@ -1704,8 +1704,20 @@ def run_event_loop():
             loop.run_until_complete(stopping_fut)
         finally:
             # clean-up
+            try:
+                pending_tasks = asyncio.gather(*asyncio.all_tasks(loop), return_exceptions=True)
+                pending_tasks.cancel()
+                with suppress(asyncio.CancelledError):
+                    loop.run_until_complete(pending_tasks)
+                loop.run_until_complete(loop.shutdown_asyncgens())
+                if isinstance(loop, asyncio.BaseEventLoop):
+                    loop.run_until_complete(loop.shutdown_default_executor())
+            except Exception as e:
+                _logger.debug(f"exception when cleaning up asyncio event loop: {e}")
+
             global _asyncio_event_loop
             _asyncio_event_loop = None
+            loop.close()
 
     loop.set_exception_handler(on_exception)
     _set_custom_task_factory(loop)