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/context_manager/history.rs

@@ -18,6 +18,7 @@ use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::InputModality;
 use codex_protocol::protocol::TokenUsage;
 use codex_protocol::protocol::TokenUsageInfo;
+use codex_protocol::protocol::TurnContextItem;
 use std::ops::Deref;
 
 /// Transcript of thread history
@@ -26,6 +27,12 @@ pub(crate) struct ContextManager {
     /// The oldest items are at the beginning of the vector.
     items: Vec<ResponseItem>,
     token_info: Option<TokenUsageInfo>,
+    /// Previous turn context snapshot used for diffing context and producing
+    /// model-visible settings update items.
+    ///
+    /// When this is `None`, settings diffing treats the next turn as having no
+    /// baseline and emits a full reinjection of context state.
+    previous_context_item: Option<TurnContextItem>,
 }
 
 #[derive(Debug, Clone, Copy, Default)]
@@ -41,6 +48,7 @@ impl ContextManager {
         Self {
             items: Vec::new(),
             token_info: TokenUsageInfo::new_or_append(&None, &None, None),
+            previous_context_item: None,
         }
     }
 
@@ -52,6 +60,14 @@ impl ContextManager {
         self.token_info = info;
     }
 
+    pub(crate) fn set_previous_context_item(&mut self, item: Option<TurnContextItem>) {
+        self.previous_context_item = item;
+    }
+
+    pub(crate) fn previous_context_item(&self) -> Option<TurnContextItem> {
+        self.previous_context_item.clone()
+    }
+
     pub(crate) fn set_token_usage_full(&mut self, context_window: i64) {
         match &mut self.token_info {
             Some(info) => info.fill_to_context_window(context_window),

codex-rs/core/src/context_manager/updates.rs

@@ -6,26 +6,27 @@ use codex_protocol::config_types::Personality;
 use codex_protocol::models::DeveloperInstructions;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ModelInfo;
+use codex_protocol::protocol::TurnContextItem;
 
 fn build_environment_update_item(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     next: &TurnContext,
     shell: &Shell,
 ) -> Option<ResponseItem> {
     let prev = previous?;
-    let prev_context = EnvironmentContext::from_turn_context(prev, shell);
+    let prev_context = EnvironmentContext::from_turn_context_item(prev, shell);
     let next_context = EnvironmentContext::from_turn_context(next, shell);
     if prev_context.equals_except_shell(&next_context) {
         return None;
     }
 
-    Some(ResponseItem::from(EnvironmentContext::diff(
-        prev, next, shell,
-    )))
+    Some(ResponseItem::from(
+        EnvironmentContext::diff_from_turn_context_item(prev, next, shell),
+    ))
 }
 
 fn build_permissions_update_item(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     next: &TurnContext,
     exec_policy: &Policy,
 ) -> Option<ResponseItem> {
@@ -46,11 +47,11 @@ fn build_permissions_update_item(
 }
 
 fn build_collaboration_mode_update_item(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     next: &TurnContext,
 ) -> Option<ResponseItem> {
     let prev = previous?;
-    if prev.collaboration_mode != next.collaboration_mode {
+    if prev.collaboration_mode.as_ref() != Some(&next.collaboration_mode) {
         // If the next mode has empty developer instructions, this returns None and we emit no
         // update, so prior collaboration instructions remain in the prompt history.
         Some(DeveloperInstructions::from_collaboration_mode(&next.collaboration_mode)?.into())
@@ -60,7 +61,7 @@ fn build_collaboration_mode_update_item(
 }
 
 fn build_personality_update_item(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     next: &TurnContext,
     personality_feature_enabled: bool,
 ) -> Option<ResponseItem> {
@@ -68,7 +69,7 @@ fn build_personality_update_item(
         return None;
     }
     let previous = previous?;
-    if next.model_info.slug != previous.model_info.slug {
+    if next.model_info.slug != previous.model {
         return None;
     }
 
@@ -96,12 +97,11 @@ pub(crate) fn personality_message_for(
 }
 
 pub(crate) fn build_model_instructions_update_item(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     resumed_model: Option<&str>,
     next: &TurnContext,
 ) -> Option<ResponseItem> {
-    let previous_model =
-        resumed_model.or_else(|| previous.map(|prev| prev.model_info.slug.as_str()))?;
+    let previous_model = resumed_model.or_else(|| previous.map(|prev| prev.model.as_str()))?;
     if previous_model == next.model_info.slug {
         return None;
     }
@@ -115,7 +115,7 @@ pub(crate) fn build_model_instructions_update_item(
 }
 
 pub(crate) fn build_settings_update_items(
-    previous: Option<&TurnContext>,
+    previous: Option<&TurnContextItem>,
     resumed_model: Option<&str>,
     next: &TurnContext,
     shell: &Shell,