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/app-server/src/codex_message_processor.rs

@@ -7323,6 +7323,7 @@ impl CodexMessageProcessor {
                 &request_id,
                 thread.as_ref(),
                 Op::RealtimeConversationStart(ConversationStartParams {
+                    output_modality: params.output_modality,
                     prompt: params.prompt,
                     session_id: params.session_id,
                     transport: params.transport.map(|transport| match transport {