Move previous turn context tracking into ContextManager history (#12179)

## Summary
- add `previous_context_item: Option<TurnContextItem>` to
`ContextManager`
- expose session/state accessors for reading and updating the stored
previous context item
- switch settings diffing to use `TurnContextItem` instead of
`TurnContext`
- remove submission-loop local `previous_context` and persist the
previous context item in history

## Testing
- `just fmt`
- `just fix -p codex-core`
- `cargo test -p codex-core --test all model_switching::`
- `cargo test -p codex-core --test all collaboration_instructions::`
- `cargo test -p codex-core --test all personality::`
- `cargo test -p codex-core --test all
permissions_messages::permissions_message_not_added_when_no_change`

codex-rs/core/src/compact.rs

@@ -118,13 +118,7 @@ async fn run_compact_task_inner(
     // Reuse one client session so turn-scoped state (sticky routing, websocket append tracking)
     // survives retries within this compact turn.
 
-    // TODO: If we need to guarantee the persisted mode always matches the prompt used for this
-    // turn, capture it in TurnContext at creation time. Using SessionConfiguration here avoids
-    // duplicating model settings on TurnContext, but an Op after turn start could update the
-    // session config before this write occurs.
-    let collaboration_mode = sess.current_collaboration_mode().await;
-    let rollout_item =
-        RolloutItem::TurnContext(turn_context.to_turn_context_item(collaboration_mode));
+    let rollout_item = RolloutItem::TurnContext(turn_context.to_turn_context_item());
     sess.persist_rollout_items(&[rollout_item]).await;
 
     loop {