code-mode: introduce durable session interface (#24180)

## Summary

Introduce a `CodeModeSession` interface for executing and managing
code-mode cells.

This moves cell lifecycle, callback delegation, termination, and
shutdown behind a session abstraction, while continuing to use the
existing in-process implementation, and the ability to implement an
external process one behind this interface.

A Codex session owns one `CodeModeSession`, which in turn owns its
running cells and stored code-mode state. Each cell is represented to
the caller as a `StartedCell`, exposing its cell ID and initial
response.

It also introduces a `CodeModeSessionDelegate` callback interface. A
session uses the delegate to invoke nested host tools and emit
notifications while a cell is running, allowing the runtime to
communicate with its owning Codex session without depending directly on
core turn handling.

<img width="2121" height="1001" alt="image"
src="https://github.com/user-attachments/assets/c349a819-2a59-485c-bda4-2caf68ac4c31"
/>

codex-rs/core/src/session/handlers.rs

@@ -630,6 +630,9 @@ async fn shutdown_session_runtime(sess: &Arc<Session>) {
         .unified_exec_manager
         .terminate_all_processes()
         .await;
+    if let Err(err) = sess.services.code_mode_service.shutdown().await {
+        warn!("failed to shutdown code mode session: {err}");
+    }
     let mcp_shutdown = {
         let mut manager = sess.services.mcp_connection_manager.write().await;
         manager.begin_shutdown()