Launch `WatchtowerPlugin.watchtower.start_watching()` through
asyncio.run_coroutine_threadsafe instead of ensure_future to prevent the
following exception from happening when running python with
`PYTHONASYNCIODEBUG=1`.
```
20251103T165007.087746Z | ERROR | plugin.Plugins | cannot initialize plugin watchtower: Error loading watchtower plugin: RuntimeError('Non-thread-safe operation invoked on an event loop other than the current one')
Traceback (most recent call last):
File "/home/user/code/electrum-fork/electrum/plugin.py", line 629, in load_plugin_by_name
plugin = module.Plugin(self, self.config, name)
File "/home/user/code/electrum-fork/electrum/plugins/watchtower/watchtower.py", line 59, in __init__
asyncio.ensure_future(self.watchtower.start_watching())
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib64/python3.14/asyncio/tasks.py", line 732, in ensure_future
return loop.create_task(coro_or_future)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
File "/usr/lib64/python3.14/asyncio/base_events.py", line 468, in create_task
return self._task_factory(self, coro, **kwargs)
~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
File "/home/user/code/electrum-fork/electrum/util.py", line 1773, in factory
task = asyncio.Task(coro, loop=loop_, **kwargs)
File "/usr/lib64/python3.14/asyncio/base_events.py", line 829, in call_soon
self._check_thread()
~~~~~~~~~~~~~~~~~~^^
File "/usr/lib64/python3.14/asyncio/base_events.py", line 866, in _check_thread
raise RuntimeError(
"Non-thread-safe operation invoked on an event loop other "
"than the current one")
RuntimeError: Non-thread-safe operation invoked on an event loop other than the current one
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "/home/user/code/electrum-fork/electrum/plugin.py", line 184, in load_plugins
self.load_plugin_by_name(name)
~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
File "/home/user/code/electrum-fork/electrum/plugin.py", line 631, in load_plugin_by_name
raise Exception(f"Error loading {name} plugin: {repr(e)}") from e
Exception: Error loading watchtower plugin: RuntimeError('Non-thread-safe operation invoked on an event loop other than the current one')
```
