Add realtime output modality and transcript events (#17701)

- Add outputModality to thread/realtime/start and wire text/audio output
selection through app-server, core, API, and TUI.\n- Rename the realtime
transcript delta notification and add a separate transcript done
notification that forwards final text from item done without correlating
it with deltas.

codex-rs/tui/src/app_server_session.rs

@@ -657,6 +657,7 @@ impl AppServerSession {
                 request_id,
                 params: ThreadRealtimeStartParams {
                     thread_id: thread_id.to_string(),
+                    output_modality: params.output_modality,
                     prompt: params.prompt,
                     session_id: params.session_id,
                     voice: params.voice,