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/app-server-protocol/src/protocol/thread_history.rs

@@ -400,7 +400,7 @@ impl ThreadHistoryBuilder {
         match item {
             RolloutItem::EventMsg(event) => self.handle_event(event),
             RolloutItem::Compacted(payload) => self.handle_compacted(payload),
-            RolloutItem::ResponseItem(item) => self.handle_response_item(item),
+            RolloutItem::ResponseItem(item) => self.handle_response_item(&item.item),
             RolloutItem::InterAgentCommunication(_)
             | RolloutItem::InterAgentCommunicationMetadata { .. }
             | RolloutItem::TurnContext(_)
@@ -4399,7 +4399,7 @@ mod tests {
                 local_images: Vec::new(),
                 ..Default::default()
             })),
-            RolloutItem::ResponseItem(hook_prompt),
+            RolloutItem::ResponseItem(hook_prompt.into()),
             RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: "turn-a".into(),
                 started_at: None,
@@ -4475,15 +4475,18 @@ mod tests {
                 model_context_window: None,
                 collaboration_mode_kind: Default::default(),
             })),
-            RolloutItem::ResponseItem(codex_protocol::models::ResponseItem::Message {
-                id: Some(codex_protocol::ResponseItemId::with_suffix("msg", "1")),
-                role: "user".into(),
-                content: vec![codex_protocol::models::ContentItem::InputText {
-                    text: "plain text".into(),
-                }],
-                phase: None,
-                internal_chat_message_metadata_passthrough: None,
-            }),
+            RolloutItem::ResponseItem(
+                codex_protocol::models::ResponseItem::Message {
+                    id: Some(codex_protocol::ResponseItemId::with_suffix("msg", "1")),
+                    role: "user".into(),
+                    content: vec![codex_protocol::models::ContentItem::InputText {
+                        text: "plain text".into(),
+                    }],
+                    phase: None,
+                    internal_chat_message_metadata_passthrough: None,
+                }
+                .into(),
+            ),
             RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: "turn-a".into(),
                 started_at: None,