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/lib.rs

@@ -35,7 +35,6 @@ pub use codex_thread::TryStartTurnIfIdleRejectionReason;
 pub use environment_config::EnvironmentConfig;
 pub use session::turn_context::TurnContext;
 pub use user_message_admission::UserMessageAdmission;
-pub use user_message_admission::UserMessageAdmissionError;
 mod agent;
 mod agent_communication;
 mod attestation;