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/session/mod.rs

@@ -229,6 +229,7 @@ mod token_budget;
 pub(crate) mod turn;
 pub(crate) mod turn_context;
 mod turn_input;
+mod turn_suspension;
 mod world_state;
 use self::code_mode_warning::unsupported_code_mode_warning;
 #[cfg(test)]