Simplify queued user message admission (#38092)

## What changed

- Resolve user message admission when Core accepts the input as a new turn or a steer, without waiting for rollout persistence.
- Remove persistence- and hook-specific admission errors and task bookkeeping.
- Delete queued messages after Core accepts them for turn processing, including messages subsequently stopped by a prompt hook.

## Testing

- Update concurrent admission and queue service tests for acceptance-based admission.

GitOrigin-RevId: 7dcc311cfecd3d710733b420f2add546edb86153

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

@@ -4124,10 +4124,6 @@ impl Session {
                 pending_input,
             )
             .await;
-        if let Some(client_id) = client_user_message_id.as_deref() {
-            self.pending_user_message_admissions
-                .associate_steered_by_client_id(client_id, active_turn_id);
-        }
         Ok(active_turn_id.clone())
     }