Retain client developer messages across context compaction (#38445)

## Why

Client-authored developer instructions should remain available after a context
window is compacted when `retain_client_developer_messages` is enabled.

## What changed

- Preserve annotated client-authored developer messages during remote compaction
  v2 and local token-budget context resets.
- Apply the existing retained-message token budget when carrying those messages
  into the new context window.

## Testing

- Cover enabled and disabled retention for remote compaction v2, manual
  token-budget compaction, and mid-turn automatic compaction.

GitOrigin-RevId: 4cb3782cb86ef46458413a17a3255d0016fd675f

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

@@ -3695,6 +3695,23 @@ impl Session {
         world_state: Arc<WorldState>,
     ) -> u64 {
         let turn_context = step_context.turn.as_ref();
+        let retained_client_developer_messages =
+            if self.enabled(Feature::RetainClientDeveloperMessages) {
+                let history = self.clone_history().await;
+                crate::compact_remote_v2::truncate_retained_messages_for_remote_compaction(
+                    history
+                        .annotated_items()
+                        .iter()
+                        .filter(|item| {
+                            crate::compact_remote_v2::is_client_authored_developer_message(item)
+                        })
+                        .cloned()
+                        .collect(),
+                    crate::compact_remote_v2::RETAINED_MESSAGE_TOKEN_BUDGET,
+                )
+            } else {
+                Vec::new()
+            };
         let window = {
             let mut state = self.state.lock().await;
             state.start_new_context_window()
@@ -3705,6 +3722,7 @@ impl Session {
             .await
             .into_iter()
             .map(ResponseItemEnvelope::new)
+            .chain(retained_client_developer_messages)
             .collect();
         let turn_context_item = turn_context.to_turn_context_item();
         self.replace_compacted_history(