Wire realtime api to core (#12268)

- Introduce `RealtimeConversationManager` for realtime API management 
- Add `op::conversation` to start conversation, insert audio, insert
text, and close conversation.
- emit conversation lifecycle and realtime events.
- Move shared realtime payload types into codex-protocol and add core
e2e websocket tests for start/replace/transport-close paths.

Things to consider:
- Should we use the same `op::` and `Events` channel to carry audio? I
think we should try this simple approach and later we can create
separate one if the channels got congested.
- Sending text updates to the client: we can start simple and later
restrict that.
- Provider auth isn't wired for now intentionally

codex-rs/core/src/rollout/policy.rs

@@ -122,6 +122,9 @@ fn event_msg_persistence_mode(ev: &EventMsg) -> Option<EventPersistenceMode> {
         | EventMsg::CollabCloseEnd(_)
         | EventMsg::CollabResumeEnd(_) => Some(EventPersistenceMode::Extended),
         EventMsg::Warning(_)
+        | EventMsg::RealtimeConversationStarted(_)
+        | EventMsg::RealtimeConversationRealtime(_)
+        | EventMsg::RealtimeConversationClosed(_)
         | EventMsg::ModelReroute(_)
         | EventMsg::AgentMessageDelta(_)
         | EventMsg::AgentReasoningDelta(_)