fix: handle ResultCode in get_fire_overview to prevent crash when offline

[deviantintegral]

Summary

• Fix TypeError: 'NoneType' object is not subscriptable crash when the fireplace is offline
• The GetFireOverview API returns ResultCode != 0 with WifiFireOverview: null for offline/failed/unavailable fireplaces, but the client unconditionally accessed WifiFireOverview
• Add FireOverviewResultCode enum mirroring the API's 5 result codes (Successful, FireOffline, Failed, FireNoLongerAvailable, UpdatingFirmware)
• Add FireUnavailableError exception (subclass of FlameConnectError) carrying result_code and optional Fire metadata from FireDetails
• Add guard clause in get_fire_overview to check ResultCode before accessing WifiFireOverview, matching the official app's behavior
• Extract shared _parse_fire helper to deduplicate Fire construction across get_fires, get_fire_overview, and the error path
• Backward compatible: responses omitting ResultCode default to success

Test plan

• All non-success ResultCode values (1-4) raise FireUnavailableError with correct code
• FireUnavailableError.fire populated from FireDetails when present, None otherwise
• Malformed FireDetails degrades gracefully to fire=None
• Omitted ResultCode defaults to success (backward compatibility)
• Unknown ResultCode values (e.g., 99) raise FireUnavailableError
• Success path (ResultCode == 0) returns FireOverview identically to before
• turn_on and turn_off propagate FireUnavailableError
• _parse_fire helper works with complete and minimal data
• All 1221 tests pass, mypy strict, ruff clean

Refs: deviantintegral/flame_connect_ha#55

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