Add realtime voice selection (#17176)

- Add realtime voice selection for realtime/start.
- Expose the supported v1/v2 voice lists and cover explicit, configured,
default, and invalid voice paths.

codex-rs/core/src/config/mod.rs

@@ -2098,6 +2098,7 @@ impl Config {
                     version: realtime.version.unwrap_or_default(),
                     session_type: realtime.session_type.unwrap_or_default(),
                     transport: realtime.transport.unwrap_or_default(),
+                    voice: realtime.voice,
                 }),
             experimental_realtime_ws_backend_prompt: cfg.experimental_realtime_ws_backend_prompt,
             experimental_realtime_ws_startup_context: cfg.experimental_realtime_ws_startup_context,