Add unfinished root turn suspension (#40038)

## Why

An active root turn needs to be stopped without marking it complete or aborted
before another runtime can recover the same turn ID.

## What changed

- Add `CodexThread::suspend_turn_and_shutdown` and `SuspendTurnOutcome`.
- Flush history, stop the active regular task, close the history writer, and
  shut down the session without recording a terminal turn event.
- Reject suspension when no supported turn is active or the loaded agent
  subtree still contains a live descendant.

## Testing

- Verify that suspension preserves unfinished history and allows the turn to be
  recovered under its original ID after the descendant guard is cleared.

GitOrigin-RevId: 1f9b019d07c51474ec2d991d263bc15cdd4f89ad

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

@@ -63,7 +63,7 @@ pub(crate) use user_shell::UserShellCommandMode;
 pub(crate) use user_shell::UserShellCommandTask;
 pub(crate) use user_shell::execute_user_shell_command;
 
-const GRACEFULL_INTERRUPTION_TIMEOUT_MS: u64 = 100;
+pub(crate) const GRACEFULL_INTERRUPTION_TIMEOUT_MS: u64 = 100;
 const TASK_COMPACT_METRIC: &str = "codex.task.compact";
 static ACTIVE_TURNS: Gauge = Gauge::new("core.turns.active");