ref: full state refactor (#4174)

## Current State Observations
- `Session` currently holds many unrelated responsibilities (history,
approval queues, task handles, rollout recorder, shell discovery, token
tracking, etc.), making it hard to reason about ownership and lifetimes.
- The anonymous `State` struct inside `codex.rs` mixes session-long data
with turn-scoped queues and approval bookkeeping.
- Turn execution (`run_task`) relies on ad-hoc local variables that
should conceptually belong to a per-turn state object.
- External modules (`codex::compact`, tests) frequently poke the raw
`Session.state` mutex, which couples them to implementation details.
- Interrupts, approvals, and rollout persistence all have bespoke
cleanup paths, contributing to subtle bugs when a turn is aborted
mid-flight.

## Desired End State
- Keep a slim `Session` object that acts as the orchestrator and façade.
It should expose a focused API (submit, approvals, interrupts, event
emission) without storing unrelated fields directly.
- Introduce a `state` module that encapsulates all mutable data
structures:
- `SessionState`: session-persistent data (history, approved commands,
token/rate-limit info, maybe user preferences).
- `ActiveTurn`: metadata for the currently running turn (sub-id, task
kind, abort handle) and an `Arc<TurnState>`.
- `TurnState`: all turn-scoped pieces (pending inputs, approval waiters,
diff tracker, review history, auto-compact flags, last agent message,
outstanding tool call bookkeeping).
- Group long-lived helpers/managers into a dedicated `SessionServices`
struct so `Session` does not accumulate "random" fields.
- Provide clear, lock-safe APIs so other modules never touch raw
mutexes.
- Ensure every turn creates/drops a `TurnState` and that
interrupts/finishes delegate cleanup to it.

codex-rs/core/src/codex/compact.rs

@@ -151,18 +151,12 @@ async fn run_compact_task_inner(
     if remove_task_on_completion {
         sess.remove_task(&sub_id).await;
     }
-    let history_snapshot = {
-        let state = sess.state.lock().await;
-        state.history.contents()
-    };
+    let history_snapshot = sess.history_snapshot().await;
     let summary_text = get_last_assistant_message_from_turn(&history_snapshot).unwrap_or_default();
     let user_messages = collect_user_messages(&history_snapshot);
     let initial_context = sess.build_initial_context(turn_context.as_ref());
     let new_history = build_compacted_history(initial_context, &user_messages, &summary_text);
-    {
-        let mut state = sess.state.lock().await;
-        state.history.replace(new_history);
-    }
+    sess.replace_history(new_history).await;
 
     let rollout_item = RolloutItem::Compacted(CompactedItem {
         message: summary_text.clone(),
@@ -270,8 +264,7 @@ async fn drain_to_completed(
         };
         match event {
             Ok(ResponseEvent::OutputItemDone(item)) => {
-                let mut state = sess.state.lock().await;
-                state.history.record_items(std::slice::from_ref(&item));
+                sess.record_into_history(std::slice::from_ref(&item)).await;
             }
             Ok(ResponseEvent::Completed { .. }) => {
                 return Ok(());