Fix attribute error on network outage

[MartinHjelmare]

Since the connection may not be finished when connect_async returns, wait for the connected event before subscribing.

Fixes the following stack trace:

Error in subscription
Traceback (most recent call last):
  File "/usr/local/lib/python3.14/site-packages/tibber/home.py", line 456, in _start_listen
    async for _data in self._tibber_control.realtime.subscribe(
    ...<5 lines>...
        self._handle_subscription_data(_data)
  File "/usr/local/lib/python3.14/site-packages/tibber/realtime.py", line 176, in subscribe
    async for result in self._session.subscribe(request):
        yield result
  File "/usr/local/lib/python3.14/site-packages/gql/client.py", line 1426, in subscribe
    async for result in inner_generator:
    ...<13 lines>...
                yield result.data
  File "/usr/local/lib/python3.14/site-packages/gql/client.py", line 2071, in _subscribe
    async for result in inner_generator:
        yield result
  File "/usr/local/lib/python3.14/site-packages/gql/client.py", line 1337, in _subscribe
    async for result in inner_generator:
    ...<11 lines>...
        yield result
  File "/usr/local/lib/python3.14/site-packages/gql/transport/common/base.py", line 281, in subscribe
    query_id: int = await self._send_query(
                    ^^^^^^^^^^^^^^^^^^^^^^^
        request,
        ^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.14/site-packages/gql/transport/websockets_protocol.py", line 244, in _send_query
    if self.subprotocol == self.GRAPHQLWS_SUBPROTOCOL:
       ^^^^^^^^^^^^^^^^
AttributeError: 'TibberWebsocketsTransport' object has no attribute 'subprotocol'