Preserve harness metadata across conversation history (#38058)

## What changed

- Wrap response items with optional harness-owned metadata while keeping the persisted response payload shape backward compatible.
- Store compacted-history metadata in an aligned sidecar and reject malformed sidecars.
- Carry metadata through history normalization, compaction, resume, fork, truncation, migration, and rollout recording without sending it to model providers.

## Testing

- Cover legacy rollout compatibility, metadata serialization and validation, compaction retention, and resume/fork reconstruction.

GitOrigin-RevId: 17a6127c9048ff984cb4df4d97c0751a38c29f5f

codex-rs/external-agent-migration/src/sessions/append.rs

@@ -220,7 +220,7 @@ fn source_model_items(items: &[RolloutItem]) -> Option<Vec<SourceModelItem<'_>>>
             RolloutItem::SessionMeta(_) | RolloutItem::InterAgentCommunicationMetadata { .. } => {}
             RolloutItem::ResponseItem(response_item) => {
                 model_items.push(SourceModelItem {
-                    response_item,
+                    response_item: &response_item.item,
                     append_start_index: append_start_index.take().unwrap_or(index),
                 });
             }
@@ -253,7 +253,7 @@ fn history_model_items(items: &[RolloutItem]) -> Option<Vec<&ResponseItem>> {
     for item in items {
         match item {
             RolloutItem::SessionMeta(_) | RolloutItem::InterAgentCommunicationMetadata { .. } => {}
-            RolloutItem::ResponseItem(response_item) => model_items.push(response_item),
+            RolloutItem::ResponseItem(response_item) => model_items.push(&response_item.item),
             RolloutItem::EventMsg(
                 EventMsg::ContextCompacted(_) | EventMsg::ThreadRolledBack(_),
             )

codex-rs/external-agent-migration/src/sessions/append_tests.rs

@@ -75,15 +75,18 @@ fn requires_a_strict_nonempty_model_prefix() {
         assert!(plan_append(&source, &rewritten).is_none());
     }
     let mut with_tool_call = history;
-    with_tool_call.push(RolloutItem::ResponseItem(ResponseItem::FunctionCall {
-        id: None,
-        name: "native_tool".to_string(),
-        namespace: None,
-        arguments: "{}".to_string(),
-        encrypted_function_args: None,
-        call_id: "native-call".to_string(),
-        internal_chat_message_metadata_passthrough: None,
-    }));
+    with_tool_call.push(RolloutItem::ResponseItem(
+        ResponseItem::FunctionCall {
+            id: None,
+            name: "native_tool".to_string(),
+            namespace: None,
+            arguments: "{}".to_string(),
+            encrypted_function_args: None,
+            call_id: "native-call".to_string(),
+            internal_chat_message_metadata_passthrough: None,
+        }
+        .into(),
+    ));
     assert!(plan_append(&source, &with_tool_call).is_none());
 }
 
@@ -105,7 +108,10 @@ fn model_messages(items: &[RolloutItem]) -> Vec<(MessageRole, &str)> {
     items
         .iter()
         .filter_map(|item| match item {
-            RolloutItem::ResponseItem(ResponseItem::Message { role, content, .. }) => {
+            RolloutItem::ResponseItem(response_item) => {
+                let ResponseItem::Message { role, content, .. } = &response_item.item else {
+                    return None;
+                };
                 match (role.as_str(), content.as_slice()) {
                     ("user", [ContentItem::InputText { text }]) => {
                         Some((MessageRole::User, text.as_str()))

codex-rs/external-agent-migration/src/sessions/export.rs

@@ -120,7 +120,7 @@ pub(super) fn rollout_items_from_messages(messages: Vec<ConversationMessage>) ->
                 )));
                 response_item_bytes =
                     response_item_bytes.saturating_add(message_byte_count(&message));
-                items.push(RolloutItem::ResponseItem(response_item(message)));
+                items.push(RolloutItem::ResponseItem(response_item(message).into()));
                 current_turn = Some((turn_id, started_at));
             }
             MessageRole::Assistant => {
@@ -137,7 +137,7 @@ pub(super) fn rollout_items_from_messages(messages: Vec<ConversationMessage>) ->
                         memory_citation: None,
                     },
                 )));
-                items.push(RolloutItem::ResponseItem(response_item(message)));
+                items.push(RolloutItem::ResponseItem(response_item(message).into()));
             }
         }
     }
@@ -325,7 +325,8 @@ mod tests {
             .filter(|item| {
                 matches!(
                     item,
-                    RolloutItem::ResponseItem(ResponseItem::Message { .. })
+                    RolloutItem::ResponseItem(response_item)
+                        if matches!(&response_item.item, ResponseItem::Message { .. })
                 )
             })
             .count();