Add push notification support for WG4-series thermostats

[adamjernst]

Refs #226 (partially: WG4 only).

What

Adds push updates for WG4-series thermostats so consumers such as Home Assistant can react to changes within seconds instead of polling.

• OJMicroline.subscribe(listener) -> unsubscribe. The first subscriber starts a background task that waits for notifications and dispatches each updated Thermostat to every listener (plain or coroutine functions). Failures are retried with exponential backoff (1 s up to 5 min); a failing listener is logged and does not affect the others. The task stops when the last subscriber leaves or the client is closed.
• OJMicrolineAPI gains supports_notifications: bool and async get_notifications() -> list[Thermostat], keeping the core client simple and the transport details in the per-model classes as discussed in Add support for notification streams #226.
• WG4API implements it. WD5 and WG5 report unsupported and raise OJMicrolineError (WD5 has a SignalR hub, see the POC in Add support for notification streams #226, and can be added in a follow-up).
• _request accepts a per-request timeout so the long-poll can outlast the default 30 s.
• README section and tests (coverage stays at 100%).

How the WG4 cloud does it

GET /api/notification?sessionid=... on mythermostat.info is a long-poll: it blocks until a thermostat changes and returns {"SequenceNr": n, "Action": 2, "Thermostat": {...full api/thermostat payload...}}, or after roughly a minute returns Action 0 with a null thermostat. Notifications are queued per session, and only once that session has called api/thermostats, so WG4API.get_notifications() re-fetches (and returns) every thermostat whenever the session id changes, which also covers the periodic re-login.

Testing

Verified live against two UWG4 thermostats: both delivered on subscribe, a setpoint change made through the API arrived in about 2 s, periodic temperature reports arrive every 25-60 s per thermostat, a forced mid-stream re-login re-subscribed and re-delivered both thermostats, and unsubscribing cleared the task.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d1701a2) to head (02d0984).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #691   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          522       608   +86     
  Branches        67        79   +12     
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+ Hits           522       608   +86     

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