core: let steer interrupt wait_agent (#28341)

## Why

`wait_agent` can block for a long timeout while waiting for sub-agent
mailbox activity. Although same-turn user steer is accepted during that
tool call, the input remains pending until the wait returns, so an
explicit request to change direction can appear unresponsive.

## What changed

- Notify active `wait_agent` calls when user input is steered into the
current turn.
- Check for already-pending steer input when subscribing so input that
races with tool startup is not missed.
- Distinguish mailbox activity, steered input, and timeout outcomes,
returning `Wait interrupted by new input.` for the steer path.
- Update the `wait_agent` tool description to document the early-return
behavior.

## Testing

- `just test -p codex-core input_queue_`
- `just test -p codex-core wait_agent`

The coverage includes steer notification before and after subscription,
plus an end-to-end test that verifies the interrupted wait result and
steered user input are both included exactly once in the follow-up model
request.

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

@@ -220,6 +220,7 @@ use self::config_lock::validate_config_lock_if_configured;
 #[cfg(test)]
 use self::handlers::submission_dispatch_span;
 use self::handlers::submission_loop;
+pub(crate) use self::input_queue::InputQueueActivity;
 pub(crate) use self::input_queue::TurnInput;
 pub(crate) use self::input_queue::TurnInputQueue;
 use self::review::spawn_review_thread;