Unify realtime v1/v2 session config (#14606)

## Summary
- unify realtime websocket settings under `[realtime]` (`version` and
`type`)
- remove `realtime_conversation_v2` and select parser/session mode from
config

## Testing
- not run (per request)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/tests/suite/v2/realtime_conversation.rs

@@ -51,7 +51,7 @@ async fn realtime_conversation_streams_v2_notifications() -> Result<()> {
         vec![],
         vec![
             json!({
-                "type": "conversation.output_audio.delta",
+                "type": "response.output_audio.delta",
                 "delta": "AQID",
                 "sample_rate": 24_000,
                 "channels": 1,
@@ -403,6 +403,10 @@ sandbox_mode = "read-only"
 model_provider = "mock_provider"
 experimental_realtime_ws_base_url = "{realtime_server_uri}"
 
+[realtime]
+version = "v2"
+type = "conversational"
+
 [features]
 {realtime_feature_key} = {realtime_enabled}