Pre-sampling compact with previous model context (#11504)

- Run pre-sampling compact through a single helper that builds
previous-model turn context and compacts before the follow-up request
when switching to a smaller context window.
- Keep compaction events on the parent turn id and add compact suite
coverage for switch-in-session and resume+switch flows.

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

@@ -153,7 +153,7 @@ impl Session {
                     if !task_cancellation_token.is_cancelled() {
                         // Emit completion uniformly from spawn site so all tasks share the same lifecycle.
                         let sess = session_ctx.clone_session();
-                        sess.on_task_finished(ctx_for_finish, last_agent_message)
+                        sess.on_task_finished(Arc::clone(&ctx_for_finish), last_agent_message)
                             .await;
                     }
                     // Set previous model regardless of completion or interruption for model-switch handling.