Add TUI realtime conversation mode (#12687)

- Add a hidden `realtime_conversation` feature flag and `/realtime`
slash command for start/stop live voice sessions.
- Reuse transcription composer/footer UI for live metering, stream mic
audio, play assistant audio, render realtime user text events, and
force-close on feature disable.

---------

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

codex-rs/tui/src/chatwidget/tests.rs

@@ -1720,6 +1720,8 @@ async fn make_chatwidget_manual(
         status_line_branch_pending: false,
         status_line_branch_lookup_complete: false,
         external_editor_state: ExternalEditorState::Closed,
+        realtime_conversation: RealtimeConversationUiState::default(),
+        last_rendered_user_message_event: None,
     };
     widget.set_model(&resolved_model);
     (widget, rx, op_rx)