Reduce TUI stack usage during session transitions (#44176)

## Why

Session transitions need stack headroom to rebuild configuration and the chat widget. Tokio stores task outputs inline even when it boxes the task future, so returning a large `Config` adds stack pressure during task allocation.

## What changed

Box the configuration build result inside the runtime worker and unbox it after joining. Box the `App::run` future to keep the large event loop out of enclosing startup futures.

## Testing

Extend the production-stack TUI integration test to enter `/side`, verify the process remains running, and return to the saved conversation with Ctrl+C.

GitOrigin-RevId: 44975300bb837bf3645ff762cf7bcdc44a277f41

codex-rs/tui/src/app/config_persistence.rs

@@ -19,8 +19,10 @@ async fn build_config_on_runtime_worker(
     builder: ConfigBuilder,
     error_context: String,
 ) -> Result<Config> {
-    match tokio::spawn(async move { builder.build().await }).await {
-        Ok(build_result) => build_result.wrap_err(error_context),
+    // Tokio stores the task output inline even when it boxes the future. Keep the large
+    // Config off the caller's stack while Tokio allocates the task during session switches.
+    match tokio::spawn(async move { builder.build().await.map(Box::new) }).await {
+        Ok(build_result) => build_result.map(|config| *config).wrap_err(error_context),
         Err(err) if err.is_panic() => std::panic::resume_unwind(err.into_panic()),
         Err(err) => Err(err).wrap_err_with(|| format!("{error_context} task failed")),
     }

codex-rs/tui/src/lib.rs

@@ -1811,7 +1811,9 @@ async fn run_ratatui_app(
         Ok(StartupHooksReviewOutcome::OpenHooksBrowser(data)) => Some(data),
     };
 
-    let app_result = App::run(
+    // Keep the large event-loop future out of the enclosing startup futures so session
+    // transitions have enough stack headroom to rebuild configuration and the chat widget.
+    let app_result = Box::pin(App::run(
         &mut tui,
         app_server,
         config,
@@ -1834,7 +1836,7 @@ async fn run_ratatui_app(
         startup_hooks_browser,
         startup_draft,
         managed_worktree,
-    )
+    ))
     .await;
 
     terminal_restore_guard.restore_silently();

codex-rs/tui/tests/suite/worktree_stack.rs

@@ -16,7 +16,7 @@ use std::time::Instant;
 use wiremock::matchers::body_string_contains;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn picker_worktree_fork_and_cd_run_on_the_production_stack() -> Result<()> {
+async fn picker_side_worktree_fork_and_cd_run_on_the_production_stack() -> Result<()> {
     let repository = tempfile::tempdir_in("/tmp")?;
     let root = repository.path().canonicalize()?;
     let status = Command::new("git")
@@ -93,6 +93,12 @@ async fn picker_worktree_fork_and_cd_run_on_the_production_stack() -> Result<()>
     terminal.wait_for_screen("STACK_SAVED_HISTORY")?;
     terminal.wait_for_screen("Ask Codex to do anything")?;
 
+    submit(&mut terminal, "/side")?;
+    terminal.wait_for_screen("Side from main thread")?;
+    terminal.ensure_running()?;
+    terminal.write_input(b"\x03")?;
+    terminal.wait_for_screen("STACK_SAVED_HISTORY")?;
+
     submit(&mut terminal, "/resume")?;
     terminal.wait_for_screen("Resume a previous session")?;
     terminal.ensure_running()?;