Add WebsocketLLMService as a base class for WebSocket-based LLM services,
parallel to WebsocketTTSService/WebsocketSTTService but codifying a
transactional request-response model rather than a continuous background
receive loop.
WebsocketLLMService provides:
- Connection lifecycle (start/stop/cancel → connect/disconnect)
- _ws_send/_ws_recv with transparent ConnectionClosed handling
(auto-reconnect via exponential backoff → WebsocketReconnectedError)
- _ensure_connected with retry via _try_reconnect
OpenAIResponsesLLMService now inherits from WebsocketLLMService, removing
duplicated connection management code (_connect, _disconnect, _reconnect,
_ensure_connected, _ws_send, start, stop, cancel) and simplifying
_process_context from a loop with attempt tracking to a flat try/except
with a single retry.
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